Lane change assist device

ABSTRACT

A first limiter limits a first target steering angle correspondence value by a first steering angle correspondence value guard which defines the upper limit value of the steering angle correspondence value and is larger than a steering angle correspondence value guard at lane change time and limits a first target steering angular velocity correspondence value by a first steering angular velocity correspondence value guard which defines the upper limit value of the steering angular velocity correspondence value and is larger than a steering angular velocity correspondence value guard at lane change time. An actuator controller for first yaw angle return control which is configured to control an actuator to operate a steering wheel so that steering angle correspondence value becomes a first target steering angle correspondence value and a steering angular velocity correspondence value becomes a first target steering angular velocity correspondence value.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a lane change assist device capable ofexecuting a lane change assist control for supporting a steeringoperation to make a lane change.

2. Description of the Related Art

Japanese Patent Application Laid-open No. 2016-126360 discloses a lanechange assist device capable of executing lane change assist controlthat supports a steering operation of a steering wheel when a vehiclemakes a lane change.

This lane change assist device can calculate a target trajectory onwhich a vehicle (hereinafter referred to as “own vehicle”) equipped witha lane change assist device travels when the own vehicle makes a lanechange from a lane on which the own vehicle is currently traveling(hereinafter referred to as “original lane”) to a lane adjacent to theoriginal lane (hereinafter referred to as “target lane”). Furthermore,the lane change assist device can control steered angles of steeredwheels of the own vehicle so that the own vehicle travels along thecalculated target trajectory.

Further, the lane change assist device determines whether or not aprobability of collision between the own vehicle and another vehicletraveling on the target lane is high when the own vehicle is moved tothe target lane along the calculated target trajectory. In other words,the lane change assist device determines whether or not a predeterminednon-permission condition is established.

Then, the lane change assist device does not execute the lane changeassist control when determining that the non-permission condition isestablished.

On the other hand, the lane change assist device executes the lanechange assist control when the non-permission condition is notestablished.

That is, in this case, the lane change assist device controls thesteered angles of the steered wheels of the own vehicle so that the ownvehicle travels along the calculated target trajectory.

SUMMARY OF THE INVENTION

Japanese Patent Application Laid-open No. 2016-126360 does not disclosea mode for controlling the own vehicle by the lane change assist devicewhen the non-permission condition is established after the lane changeassist control is started.

The present invention has been made to cope with the above problems, andhas an object to provide a lane change assist device capable ofappropriately controlling a vehicle equipped with the lane change assistdevice when a non-permission condition is established after a lanechange assist control is started.

In order to achieve the object, the lane change assist device of thepresent invention comprises:

a surrounding monitor (11) configured to monitor a surrounding of a ownvehicle (C):

a lane recognition device (10, 12) configured to recognize a compartmentline (WL) defining a side edge portion of a lane on which the ownvehicle is traveling, and to detect a relative position of the ownvehicle in a lane width direction with respect to the lane on which theown vehicle is traveling and detect a yaw angle (Gy) with respect to anextension direction of the lane on which the own vehicle is travelingbased on a positional relationship between the compartment line and theown vehicle;

an actuator (22) configured to be capable of generating a driving forcefor changing a steering angle correspondence value which is a steeringangle of a steering wheel of the own vehicle or is a torquecorresponding to the steering angle and changing a steering angularvelocity correspondence value which is a steering angular velocity beinga change amount of the steering angle per unit time or a torque changerate being a change amount of the torque corresponding to the steeringangular velocity per unit time;

a lane change assist controller (10, 20) configured to start lane changeassist control (LCA) at a predetermined lane change start time (t0),wherein the actuator is controlled under the lane change assist controlso that the own vehicle makes a lane change from an original lane onwhich the own vehicle is traveling to a target lane which is adjacent tothe original lane based on the relative position detected by the lanerecognition device;

a limiter at lane change time (10) configured to limit the steeringangle correspondence value when the lane change assist control isexecuted by a steering angle correspondence value guard at lane changetime defining an upper limit value of the steering angle correspondencevalue and to limit the steering angular velocity correspondence valuewhen the lane change assist control is executed by a steering angularvelocity correspondence value guard at lane change time defining anupper limit value of the steering angular velocity correspondence value;

a first interruption condition determiner (10) configured to make thelane change assist controller interrupt the lane change assist controlwhen a predetermined first interruption condition is established afterthe lane change assist control is started, the first interruptioncondition being established when it is determined that a probability ofthe own vehicle colliding with another vehicle travelling on the targetlane is high based on a monitoring result of the surrounding monitor;

a first target value calculator (10) configured to calculate a firsttarget steering angle correspondence value which is a target value ofthe steering angle correspondence value and a first target steeringangular velocity correspondence value which is a target value of thesteering angular velocity correspondence value, both the first targetsteering angle correspondence value and the first target steeringangular velocity correspondence value being used for executing first yawangle return control, the first yaw angle return control being startedat a predetermined first start time (t1 a) when the first interruptioncondition is established, wherein the actuator is controlled under thefirst yaw angle return control so that the yaw angle at a first finishtime (t3 a) becomes a value closer to the yaw angle at the lane changestart time compared with the yaw angle at the first start time, thefirst finish time coming when a predetermined first control executiontime (TC1) passes from the first start time;

a first limiter (10) configured to limit the first target steering anglecorrespondence value by a first steering angle correspondence valueguard (CumaxG1) which defines the upper limit value of the steeringangle correspondence value and is larger than the steering anglecorrespondence value guard at lane change time and to limit the firsttarget steering angular velocity correspondence value by a firststeering angular velocity correspondence value guard (Cu′maxG1) whichdefines the upper limit value of the steering angular velocitycorrespondence value and is larger than the steering angular velocitycorrespondence value guard at lane change time; and

an actuator controller for first yaw angle return control (20)configured to control the actuator to operate the steering wheel so thatthe steering angle correspondence value becomes the first targetsteering angle correspondence value and the steering angular velocitycorrespondence value becomes the first target steering angular velocitycorrespondence value, wherein the first target steering anglecorrespondence value and the first target steering angular velocitycorrespondence value are limited by the first limiter.

The compartment line is, for example, a white line drawn on a road.Further, the white line includes, for example, a solid line and adot-and-dash line.

In the present invention, when the lane recognition device recognizesthe compartment line defining the side edge portion of the lane, thelane recognition device detects the relative position of the own vehiclein the lane width direction with respect to the lane on which the ownvehicle is traveling based on the positional relationship between thecompartment line and the own vehicle.

Then, the lane change assist controller starts the lane change assistcontrol at the lane change start time. The actuator for changing thesteering angle of the steering wheel of the own vehicle is controlledunder the lane change assist control so that the own vehicle travellingon the original lane makes a lane change from the original lane to thetarget lane.

Further, the limiter at lane change time limits the steering anglecorrespondence value when the lane change assist control is executed bythe steering angle correspondence value guard at lane change timedefining an upper limit value of the steering angle correspondence valueand limits the steering angular velocity correspondence value when thelane change assist control is executed by the steering angular velocitycorrespondence value guard at lane change time defining an upper limitvalue of the steering angular velocity correspondence value.

After the lane change assist control is started, the first interruptioncondition determiner determines whether or not the predetermined firstinterruption condition is established based on the monitoring result ofthe surrounding monitor. The first interruption condition is establishedwhen it is determined that a probability of the own vehicle collidingwith another vehicle travelling on the target lane is high.

Then, when the first interruption condition is determined to beestablished, the first interruption condition determiner makes the lanechange assist controller interrupt the lane change assist control.

Then, when the first interruption condition is established and thus thelane change assist controller interrupts the lane change assist control,the first yaw angle return control is executed. The first yaw anglereturn control is started at the predetermined first start time. Theactuator is controlled under the first yaw angle return control so thatthe yaw angle at the first finish time, which comes when thepredetermined first control execution time passes from the first starttime, is the value closer to the yaw angle at the lane change start timecompared with the yaw angle at the first start time.

Incidentally, the first target steering angle correspondence value whichis a target value of the steering angle correspondence value forexecuting the first yaw angle return control and the first targetsteering angular velocity correspondence value which is a target valueof the steering angular velocity correspondence value for executing thefirst yaw angle return control are calculated by the first target valuecalculator.

However, considering ride comfort of an occupant of the own vehicle andthe like, when the values of the first target steering anglecorrespondence value and the first target steering angular velocitycorrespondence value are extremely large, it is not preferable toexecute the first yaw angle return control based on the first targetsteering angle correspondence value and the first target steeringangular velocity correspondence value. In contrast, when the values ofthe first target steering angle correspondence value and the firsttarget steering angular velocity correspondence value are too small, thefirst yaw angle return control cannot be quickly executed.

Therefore, the present invention comprises the first limiter.

The first limiter limits the first target steering angle correspondencevalue by the first steering angle correspondence value guard and limitsthe first target steering angular velocity correspondence value by thefirst steering angular velocity correspondence value guard.

Then, the first steering angle correspondence value guard and the firststeering angular velocity correspondence value guard, which are guardsfor the first yaw angle return control executed when a probability ofthe own vehicle colliding with another vehicle is high, are set to avalue larger than the steering angle correspondence value guard at lanechange time and a value larger than the steering angular velocitycorrespondence value guard at lane change time, respectively.

Thus, when the first yaw angle return control is executed, the firsttarget steering angle correspondence value and the first target steeringangular velocity correspondence value don't become small values.

Therefore, the first yaw angle return control can be quickly executed.

In one aspect of the present invention, the lane change assist devicefurther comprises:

a second interruption condition determiner (10) configured to make thelane change assist controller interrupt the lane change assist controlwhen a predetermined second interruption condition is established afterthe lane change assist control is started, the second interruptioncondition being established when the lane recognition device cannotdetect the relative position;

a second target value calculator (10) configured to calculate a secondtarget steering angle which is a target value of the steering angle anda second target steering angular velocity which is a target value of thesteering angular velocity, both the second target steering angle and thesecond target steering angular velocity being used for executing secondyaw angle return control, the second yaw angle return control beingstarted at a predetermined second start time (t1 b) when the secondinterruption condition is established, wherein the actuator iscontrolled under the second yaw angle return control so that the yawangle at a second finish time (T3 b) becomes a value closer to the yawangle at the lane change start time compared with the yaw angle at thesecond start time, the second finish time coming when a predeterminedsecond control execution time (TC2) which is longer than the firstcontrol execution time passes from the second start time;

a second limiter (10) configured to limit the second target steeringangle by a second steering angle guard (CumaxG2) which defines an upperlimit value of the steering angle and is smaller than the first steeringangle correspondence value guard and to limit the second target steeringangular velocity by a second steering angular velocity guard (Cu′maxG2)which defines an upper limit value of the steering angular velocity andis smaller than the first steering angular velocity correspondence valueguard; and

an actuator controller for second yaw angle return control (20)configured to control the actuator to operate the steering wheel so thatthe steering angle becomes the second target steering angle and thesteering angular velocity becomes the second target steering angularvelocity, wherein the second target steering angle and the second targetsteering angular velocity are limited by the second limiter.

The second interruption condition determiner determines whether or notthe predetermined second interruption condition is established after thelane change assist control is started. The second interruption conditionis established when the lane recognition device cannot detect therelative position of the own vehicle in the lane width direction withrespect to the lane on which the own vehicle is traveling.

When determining that the second interruption condition is established,the second interruption condition determiner makes the lane changeassist controller interrupt the lane change assist control.

When the lane change assist controller interrupts the lane change assistcontrol due to the establishment of the second interruption condition,the second yaw angle return control is executed. The second yaw anglereturn control is started at the predetermined second start time. Underthe second yaw angle return control, the actuator is controlled so thatthe yaw angle at the second finish time becomes a value closer to theyaw angle at the lane change start time compared with the yaw angle atthe second start time. The second finish time comes when thepredetermined second control execution time passes from the second starttime.

Incidentally, the second target steering angle which is a target valueof the steering angle for executing the second yaw angle return controland the second target steering angular velocity which is a target valueof the steering angular velocity for executing the second yaw anglereturn control are calculated by the second target value calculator.

However, considering ride comfort of an occupant of the own vehicle,when the values of the second target steering angle and the secondtarget steering angular velocity are large, it is not preferable toexecute the second yaw angle return control based on the second targetsteering angle and the second target steering angular velocity.

Therefore, this aspect of the present invention comprises the secondlimiter.

The second limiter limits the second target steering angle by the secondsteering angle guard and limits the second target steering angularvelocity by the second steering angular velocity guard.

The second steering angle guard and the second steering angular velocityguard, which are guards for the second yaw angle return control executedwhen a probability of the own vehicle colliding with another vehicle isnot high, are smaller than the first steering angle correspondence valueguard and the first steering angular velocity correspondence valueguard, which are guards for the first yaw angle return control executedwhen the probability of the own vehicle colliding with another vehicleis high, respectively.

Thus, when the second yaw angle return control is executed, the secondtarget steering angle and the second target steering angular velocitydon't become large values. Therefore, during execution of the second yawangle return control, ride comfort of an occupant of the own vehicle ishard to get worse.

In the above description, references used in the following descriptionsregarding embodiments are added with parentheses to the elements of thepresent invention, in order to understand the invention. However, thosereferences should not be used to limit the scope of the presentinvention.

Other objects, other features, and accompanying advantages of thepresent invention are easily understood from the description ofembodiments of the present invention to be given referring to thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram for illustrating a steeringassist device according to an embodiment of the present invention.

FIG. 2 is a plan view for illustrating mounting positions of surroundingsensors and a camera sensor.

FIG. 3 is a diagram for illustrating lane-related vehicle information.

FIG. 4 is a diagram for illustrating actuation of a turn signal lever.

FIG. 5 is a flowchart for illustrating a steering assist controlroutine.

FIG. 6 is a flowchart for illustrating a subroutine A.

FIG. 7 is a flowchart for illustrating a subroutine B.

FIG. 8 is a diagram for illustrating an LTA screen and an LCA screen ofa display unit.

FIG. 9 is a diagram for illustrating a target trajectory of an ownvehicle.

FIG. 10 is a diagram for illustrating a target trajectory function.

FIG. 11 is a graph for showing a target curvature when a first yaw anglereturn control is executed.

FIG. 12 is a diagram for illustrating the screen of the display unitwhen the first yaw angle return control is executed.

FIG. 13 is a diagram for illustrating the target trajectory and anoriginal lane return trajectory.

FIG. 14 is a graph for illustrating a target curvature when a second yawangle return control is executed.

FIG. 15 is a diagram for illustrating the screen of the display unitwhen the second yaw angle return control is executed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, a steering assist device for avehicle according to an embodiment of the present invention is describedbelow.

The steering assist device according to the embodiment of the presentinvention is applied to a vehicle (hereinafter it may be also referredto as “own vehicle” in order to distinguish from other vehicles), and asillustrated in FIG. 1, includes a driving support ECU 10, an electricpower steering ECU 20, a meter ECU 30, a steering ECU 40, an engine ECU50, a brake ECU 60, and a navigation ECU 70.

Those ECUs are electric control units each including a microcomputer asa main part, and are connected to one another so as to be able tomutually transmit and receive information via a controller area network(CAN) 100. The microcomputer herein includes a CPU, a ROM, a RAM, anonvolatile memory, an interface I/F, and the like. The CPU executesinstructions (programs and routines) stored in the ROM to implementvarious functions. Some or all of those ECUs may be integrated into oneECU.

A plurality of types of vehicle state sensors 80 configured to detect avehicle state and a plurality of types of driving operation statesensors 90 configured to detect a driving operation state are connectedto the CAN 100. Examples of the vehicle state sensors 80 include avehicle speed sensor configured to detect a travel speed of the vehicle,a front-rear G sensor configured to detect an acceleration of thevehicle in a front-rear direction, a lateral G sensor configured todetect an acceleration of the vehicle in a lateral direction, and a yawrate sensor configured to detect a yaw rate of the vehicle.

Examples of the driving operation state sensors 90 include anaccelerator operation amount sensor configured to detect an operationamount of an accelerator pedal, a brake operation amount sensorconfigured to detect an operation amount of a brake pedal, a brakeswitch configured to detect presence or absence of the operation on thebrake pedal, a steering angle sensor configured to detect a steeringangle, a steering torque sensor configured to detect a steering torque,and a shift position sensor configured to detect a shift position of atransmission.

Information (called “sensor information”) detected by the vehicle statesensors 80 and the driving operation state sensors 90 is transmitted tothe CAN 100. In each ECU, the sensor information transmitted to the CAN100 can be used as appropriate. The sensor information is information ofa sensor connected to a specific ECU, and may be transmitted from thespecific ECU to the CAN 100. For example, the accelerator operationamount sensor may be connected to the engine ECU 50. In this case, thesensor information representing the accelerator operation amount istransmitted from the engine ECU 50 to the CAN 100. For example, thesteering angle sensor may be connected to the steering ECU 40. In thiscase, the sensor information representing the steering angle istransmitted from the steering ECU 40 to the CAN 100. The same applies tothe other sensors. Further, there may be employed a configuration inwhich, without interpolation of the CAN 100, the sensor information istransmitted and received through direct communication between specificECUs.

The driving support ECU 10 is a control device serving as a main devicefor performing driving support for a driver, and executes lane changeassist control, lane trace assist control, and adaptive cruise control.As illustrated in FIG. 2, a front-center surrounding sensor 11FC, afront-right surrounding sensor 11FR, a front-left surrounding sensor11FL, a rear-right surrounding sensor 11RR, and a rear-left surroundingsensor 11RL are connected to the driving support ECU 10. The surroundingsensors 11FC, 11FR, 11FL, 11RR, and 11RL are radar sensors, andbasically have the same configuration except that the sensors havedifferent detection regions. In the following, the surrounding sensors11FC, 11FR, 11FL, 11RR, and 11RL are called “surrounding sensors 11”when the sensors are not required to be individually distinguished fromone another.

Each of the surrounding sensors 11 includes a radar transceiver and asignal processor (not shown). The radar transceiver radiates a radiowave in a millimeter waveband (hereinafter referred to as “millimeterwave”), and receives a millimeter wave (that is, reflected wave)reflected by a three-dimensional object (e.g., other vehicles,pedestrian, bicycle, and building) present within a radiation range. Thesignal processor acquires, every time a predetermined time periodelapses, information (hereinafter called “surrounding information”)representing, for example, a distance between the own vehicle and thethree-dimensional object, a relative speed between the own vehicle andthe three-dimensional object, and a relative position (direction) of thethree-dimensional object with respect to the own vehicle based on, forexample, a phase difference between the transmitted millimeter wave andthe received reflected wave, an attenuation level of the reflected wave,and a time period required from transmission of the millimeter wave toreception of the reflected wave. Then, the signal processor transmitsthe surrounding information to the driving support ECU 10. Thesurrounding information can be used to detect a front-rear directioncomponent and a lateral direction component in the distance between theown vehicle and the three-dimensional object and a front-rear directioncomponent and a lateral direction component in the relative speedbetween the own vehicle and the three-dimensional object.

As illustrated in FIG. 2, the front-center surrounding sensor 11FC isprovided at a front-center portion of a vehicle body, and detects athree-dimensional object present in a front region of the own vehicle.The front-right surrounding sensor 11FR is provided at a front-rightcorner portion of the vehicle body, and mainly detects athree-dimensional object present in a front-right region of the ownvehicle. The front-left surrounding sensor 11FL is provided at afront-left corner portion of the vehicle body, and mainly detects athree-dimensional object present in a front-left region of the ownvehicle. The rear-right surrounding sensor 11RR is provided at arear-right corner portion of the vehicle body, and mainly detects athree-dimensional object present in a rear-right region of the ownvehicle. The rear-left surrounding sensor 11RL is provided at arear-left corner portion of the vehicle body, and mainly detects athree-dimensional object present in a rear-left region of the ownvehicle.

In this embodiment, the surrounding sensors 11 are radar sensors, butother sensors such as clearance sonars and light detection and ranging(LIDAR) sensors can be employed instead.

Further, a camera sensor 12 is connected to the driving support ECU 10.The camera sensor 12 includes a camera unit and a lane recognition unitconfigured to analyze image data obtained based on an image taken by thecamera unit to recognize a white line of a road. The camera sensor 12(camera unit) photographs a landscape in front of the own vehicle. Thecamera sensor 12 (lane recognition unit) repeatedly supplies informationrelating to the recognized white line to the driving support ECU 10 atpredetermined calculation periods.

The camera sensor 12 is capable of recognizing a lane representing aregion sectioned by white lines, and detecting a relative positionalrelation of the own vehicle with respect to the lane based on apositional relation between the white lines and the own vehicle. Theposition of the own vehicle is the center of gravity of the own vehicle.A lateral position, which is described later, of the own vehiclerepresents the position of the center of gravity of the own vehicle inthe lane width direction, a lateral speed of the own vehicle representsthe speed of the center of gravity of the own vehicle in the lane widthdirection, and a lateral acceleration of the own vehicle represents theacceleration of the center of gravity of the own vehicle in the lanewidth direction. The lateral position, the lateral speed, and thelateral acceleration are determined based on the relative positionalrelation between the white lines detected by the camera sensor 12 andthe own vehicle. In this embodiment, the position of the own vehicle isthe center of gravity. However, the position of the own vehicle is notlimited to the center of gravity, and a specific position set in advance(e.g., a center position in a planar view) can also be used as theposition of the own vehicle.

As illustrated in FIG. 3, the camera sensor 12 determines a lane centerline CL corresponding to a center position of a lane on which the ownvehicle is traveling in a width direction of right and left white linesWL. The lane center line CL is used as a target travel line in the lanetrace assist control to be described later. Further, the camera sensor12 calculates a curvature Cu of a curve of the lane center line CL.

The camera sensor 12 also calculates the position and the direction ofthe own vehicle in the lane sectioned by the right and left white linesWL. For example, as illustrated in FIG. 3, the camera sensor 12calculates a distance Dy (m) in a lane width direction between a centerof gravity point P of an own vehicle C and the lane center line CL,namely, the distance Dy by which the own vehicle C is shifted from thelane center line CL in the lane width direction. This distance Dy isreferred to as “lateral deviation Dy”. The camera sensor 12 alsocalculates an angle formed between the direction of the lane center lineCL and the direction in which the own vehicle C faces, namely, an angleθy (rad) by which the direction in which the own vehicle C faces isshifted in a horizontal direction from the direction of the lane centerline CL. This angle θy is referred to as “yaw angle θy”. When the laneis curved, the lane center line CL is also curved, and thus the yawangle θy represents the angle by which the direction in which the ownvehicle C faces is shifted from the curved lane center line CL. In thefollowing, information (Cu, Dy, and θy) representing the curvature Cu,the lateral deviation Dy, and the yaw angle θy is referred to as“lane-related vehicle information”. The right and left directions of thelateral deviation Dy and the yaw angle θy with respect to the lanecenter line CL are identified by a sign (plus or minus). Regarding thecurvature Cu, the direction of the curve (right or left) is identifiedby a sign (plus or minus).

Further, the camera sensor 12 also supplies, to the driving support ECU10, information relating to the white line, for example, the type of thedetected white line (solid line or broken line), a distance (lane width)between the right and left adjacent white lines, and the shape of thewhite line, on not only the lane on which the own vehicle is positionedbut also on adjacent lanes on predetermined calculation periods. Whenthe white line is a solid line, the vehicle is inhibited from crossingthe white line to change the lane. Otherwise, when the white line is abroken line (white line intermittently formed at certain intervals), thevehicle is allowed to cross the white line to change the lane. Thelane-related vehicle information (Cu, Dy, and θy) and the informationrelating to the white line are collectively referred to as “laneinformation”.

In this embodiment, the camera sensor 12 calculates the lane-relatedvehicle information (Cu, Dy, and θy). However, in place of the camerasensor 12, the driving support ECU 10 may acquire the lane informationby analyzing the image data output from the camera sensor 12.

Further, the camera sensor 12 can also detect a three-dimensional objectpresent in front of the own vehicle based on the image data. Therefore,not only the lane information but also front surrounding information maybe acquired through calculation. In this case, for example, there may beprovided a synthesis processor (not shown) configured to synthesize thesurrounding information acquired by the front-center surrounding sensor11FC, the front-right surrounding sensor 11FR, and the front-leftsurrounding sensor 11FL and the surrounding information acquired by thecamera sensor 12 to generate front surrounding information having a highdetection accuracy, and the surrounding information generated by thesynthesis processor may be supplied to the driving support ECU 10 as thefront surrounding information on the own vehicle.

As illustrated in FIG. 1, a buzzer 13 is connected to the drivingsupport ECU 10. The buzzer 13 produces a sound when receiving a buzzersounding signal from the driving support ECU 10. The driving support ECU10 sounds the buzzer 13 when, for example, the driving support ECU 10notifies the driver of a driving support situation, or when the drivingsupport ECU 10 alerts the driver.

In this embodiment, the buzzer 13 is connected to the driving supportECU 10, but the buzzer 13 may be connected to other ECUs, for example, anotification ECU (not shown) dedicated for notification, and the buzzer13 may be sounded by the notification ECU. In this case, the drivingsupport ECU 10 transmits a buzzer sounding command to the notificationECU.

Further, instead of or in addition to the buzzer 13, a vibrator fortransmitting vibration for notification to the driver may be provided.For example, the vibrator is provided to a steering wheel to vibrate thesteering wheel, to thereby alert the driver.

The driving support ECU 10 executes the lane change assist control, thelane trace assist control, and the adaptive cruise control based on thesurrounding information supplied from the surrounding sensors 11, thelane information obtained based on the white line recognition by thecamera sensor 12, the vehicle state detected by the vehicle statesensors 80, the driving operation state detected by the drivingoperation state sensors 90, and the like.

A setting operation unit 14 to be operated by the driver is connected tothe driving support ECU 10. The setting operation unit 14 is anoperation unit for performing setting or the like regarding whether ornot to execute each of the lane change assist control, the lane traceassist control, and the adaptive cruise control. The driving support ECU10 receives a setting signal as input from the setting operation unit 14to determine whether or not to execute each control. In this case, whenthe execution of the adaptive cruise control is not selected, the lanechange assist control and the lane trace assist control are alsoautomatically set to be unexecuted. Further, when the execution of thelane trace assist control is not selected, the lane change assistcontrol is also automatically set to be unexecuted.

Further, the setting operation unit 14 has a function of inputtingparameters or the like representing the preference of the driver whenthe above-mentioned control is executed.

The electric power steering ECU 20 is a control device for an electricpower steering device. In the following, the electric power steering ECU20 is called “EPS·ECU 20”. The EPS·ECU 20 is connected to a motor driver21. The motor driver 21 is connected to a steering motor 22. Thesteering motor 22 is integrated into a “steering mechanism including thesteering wheel, a steering shaft coupled to the steering wheel, asteering gear mechanism, and the like” (not shown) of the vehicle. TheEPS·ECU 20 detects the steering torque that is input by the driver tothe steering wheel (not shown) by a steering torque sensor provided tothe steering shaft, and controls energization to the motor driver 21based on the steering torque to drive the steering motor 22. The assistmotor is driven as described above so that the steering torque isapplied to the steering mechanism, and thus the steering operation ofthe driver is assisted.

Further, when the EPS·ECU 20 receives a steering command from thedriving support ECU 10 via the CAN 100, the EPS·ECU 20 drives thesteering motor 22 at a control amount indicated by the steering commandto generate a steering torque. This steering torque represents a torqueto be applied to the steering mechanism in response to the steeringcommand from the driving support ECU 10, which does not require thedriver's steering operation (steering wheel operation) unlike a steeringassist torque to be applied for alleviating the driver's steeringoperation described above.

Even in a case where a steering command is received from the drivingsupport ECU 10, when a steering torque from a steering wheel operationby the driver is detected and that steering torque is larger than athreshold, the EPS·ECU 20 prioritizes the steering wheel operation bythe driver and generates a steering assist torque that lightens thesteering wheel operation.

The meter ECU 30 is connected to a display unit 31 and right and leftturn signals 32 (meaning turn signal lamps and sometimes also called“turn lamps”). The display unit 31 is, for example, a multi-informationdisplay provided in front of a driver's seat, and displays various typesof information in addition to values measured by meters, for example, avehicle speed. For example, when the meter ECU 30 receives a displaycommand in accordance with the driving support state from the drivingsupport ECU 10, the meter ECU 30 causes the display unit 31 to display ascreen instructed in the display command. As the display unit 31,instead of or in addition to the multi-information display, a head-updisplay (not shown) can also be employed. When the head-up display isemployed, it is preferred to provide a dedicated ECU for controlling thedisplay on the head-up display.

Further, the meter ECU 30 includes a turn signal drive circuit (notshown). When the meter ECU 30 receives a turn signal flashing commandvia the CAN 100, the meter ECU 30 flashes the turn signal 32 arranged ina right or left direction instructed by the turn signal flashingcommand. Further, while the meter ECU 30 flashes the turn signal 32, themeter ECU 30 transmits, to the CAN 100, turn signal flashing informationrepresenting that the turn signal 32 is in a flashing state. Therefore,other ECUs can recognize the flashing state of the turn signal 32.

The steering ECU 40 is connected to a turn signal lever 41. The turnsignal lever 41 is an operation unit for actuating (flashing) the turnsignal 32, and is provided to a steering column. The turn signal lever41 is provided to be swingable at a two-stage operation stroke about asupport shaft in each of a counterclockwise operation direction and aclockwise operation direction.

The turn signal lever 41 in this embodiment also acts as an operationdevice for requesting lane change assist control by the driver. Asillustrated in FIG. 4, the turn signal lever 41 is configured to becapable of being selectively operated between a first stroke positionP1L (P1R), which is a position rotated by a first angle θW1 from aneutral position PN, and a second stroke position P2L (P2R), which is aposition rotated by a second angle θW2 (>θW1) from the neutral positionPN, in each of the clockwise operation direction and thecounterclockwise operation direction about the support shaft O. When theturn signal lever 41 is moved to the first stroke position P1L (P1R) bya lever operation by the driver, the turn signal lever 41 returns to theneutral position PN when the lever operation force by the driver isreleased. When the turn signal lever 41 is moved to the second strokeposition P2L (P2R) by a lever operation by the driver, the turn signallever 41 is held at the second stroke position P2L (P2R) by a lockmechanism even when the lever force is released. Under a state in whichthe turn signal lever 41 is held at the second stroke position P2L(P2R), when the steering wheel is reversely rotated and returned to theneutral position, or when the driver operates and returns the turnsignal lever 41 to the neutral position, the locking by the lockmechanism is released, and the turn signal lever 41 is returned to theneutral position PN.

The turn signal lever 41 includes a first switch 411L (411R) that turnson (generates an ON signal) only when the turn signal lever 41 ispositioned at the first stroke position P1L (P1R), and a second switch412L (412R) that turns on (generates an ON signal) only when the turnsignal lever 41 is positioned at the second operation position P2L(P2R).

The steering ECU 40 detects the operation state of the turn signal lever41 based on the presence/absence of the ON signal from the first switch411L (411R) and the second switch 412L (412R). When the turn signallever 41 is in a state tilted to the first stroke position P1L (P1R) andwhen the turn signal lever 41 is in a state tilted to the second strokeposition P2L (P2R), the steering ECU 40 transmits, to the meter ECU 30,a turn signal flashing command including information representing theoperation direction (right or left).

The steering ECU 40 outputs, when the turn signal lever 41 is detectedas having been continuously held at the first stroke position P1L (P1R)for a set time (lane change request confirmation time: e.g., 1 second)or more set in advance, to the driving support ECU 10 a lane changeassist request signal including information representing that operationdirection (right or left). Therefore, when the driver wishes to receivelane change assist during driving, the driver is only required to tiltthe turn signal lever 41 to the first stroke position P1L (P1R)corresponding to the lane change direction and maintain that state forthe set time or more. This operation is referred to as a lane changeassist request operation.

In this embodiment, the turn signal lever 41 is used as the operationdevice for the driver to request lane change assist. However, in placeof the turn signal lever 41, a dedicated lane change assist requestoperation device may be arranged on the steering wheel, for example.

The engine ECU 50 illustrated in FIG. 1 is connected to an engineactuator 51. The engine actuator 51 is an actuator for changing anoperation state of an internal combustion engine 52. In this embodiment,the internal combustion engine 52 is a gasoline fuel injection, sparkignition, multi-cylinder engine, and includes a throttle valve foradjusting an intake air amount. The engine actuator 51 includes at leasta throttle valve actuator for changing an opening degree of the throttlevalve. The engine ECU 50 can drive the engine actuator 51, therebychanging a torque generated by the internal combustion engine 52. Thetorque generated by the internal combustion engine 52 is transmitted todrive wheels (not shown) via a transmission (not shown). Thus, theengine ECU 50 can control the engine actuator 51 to control a drivingforce of the own vehicle, thereby changing an acceleration state(acceleration).

The brake ECU 60 is connected to a brake actuator 61. The brake actuator61 is provided in a hydraulic circuit between a master cylinder (notshown) configured to pressurize a working fluid with a stepping force ona brake pedal and friction brake mechanisms 62 provided on thefront/rear left/right wheels. The friction brake mechanism 62 includes abrake disk 62 a fixed to wheels and a brake caliper 62 b fixed to avehicle body. The brake actuator 61 is configured to adjust a hydraulicpressure supplied to a wheel cylinder built into the brake caliper 62 bin accordance with an instruction from the brake ECU 60 to use thehydraulic pressure to operate the wheel cylinder, thereby pressing abrake pad against the brake disk 62 a and generating a friction brakingforce. Thus, the brake ECU 60 can control the brake actuator 61, therebycontrolling the braking force of the own vehicle to change adeceleration state (deceleration).

The navigation ECU 70 includes a GPS receiver 71 configured to receive aGPS signal for detecting a current position of the own vehicle, a mapdatabase 72 having map information and the like stored therein, and atouch panel (touch panel-type display) 73. The navigation ECU 70identifies the position of the own vehicle at the current time pointbased on the GPS signal, and performs various types of calculationprocessing based on the position of the own vehicle and the mapinformation stored in the map database 72 and the like, to therebyperform route guidance with use of the touch panel 73.

The map information stored in the map database 72 includes roadinformation. The road information includes parameters (e.g., roadcurvature radius or curvature, the road lane width, number of roadlanes, and the position of the lane center line in each road lane)indicating the position and shape of the road. Further, the roadinformation includes road type information for enabling distinction ofwhether or not the road is a road for exclusive use by automobiles, forexample.

<Control Processing Performed by Driving Support ECU 10>

Next, control processing performed by the driving support ECU 10 isdescribed. Under a situation in which both of the lane trace assistcontrol and the adaptive cruise control are executed, when the lanechange assist request is accepted, the driving support ECU 10 executesthe lane change assist control. In view of this, the lane trace assistcontrol and the adaptive cruise control are first described.

<Lane Trace Assist Control (LTA)>

The lane trace assist control applies the steering torque to thesteering mechanism so that the position of the own vehicle is maintainedin a vicinity of the target travel line inside a “lane on which the ownvehicle is traveling”, thereby assisting the steering operation of thedriver. In this embodiment, the target travel line is the lane centerline CL, but a line offset in the lane width direction by apredetermined distance from the lane center line CL can also be adopted.

In the following, the lane trace assist control is called “LTA”. The LTAis widely known (e.g., refer to Japanese Patent Application Laid-openNo. 2008-195402, Japanese Patent Application Laid-open No. 2009-190464,Japanese Patent Application Laid-open No. 2010-6279, and Japanese PatentNo. 4349210) although the LTA itself has different names. Thus, a briefdescription is now given of the LTA.

The driving support ECU 10 is configured to carry out the LTA when theLTA is requested by the operation on the setting operation unit 14. Whenthe LTA is requested, the driving support ECU 10 calculates a targetsteering angle θlta* at a predetermined calculation cycle in accordancewith Expression (1) based on the above-mentioned lane-related vehicleinformation (Cu, Dy, and θy).

θlta*=Klta1·Cu+Klta2·θy+Klta3·Dy+Klta4·ΣDy  (1)

In Expression (1), Klta1, Klta2, Klta3, and Klta4 are control gains. Thefirst term on the right-hand side is a steering angle component that isdetermined in accordance with the curvature Cu of the road and acts in afeed-forward manner. The second term on the right-hand side is asteering angle component that acts in the feed-back manner so that theyaw angle θy is decreased (so that the difference of the direction ofthe own vehicle with respect to the lane center line CL is decreased).That is, the second term on the right-hand side is a steering anglecomponent calculated by feed-back control with the target value of theyaw angle θy being set to zero. The third term on the right-hand side isa steering angle component that acts in a feed-back manner so that thelateral difference Dy, which is a positional shift (positionaldifference) in the lane width direction of the own vehicle with respectto the lane center line CL, is decreased. That is, the third term on theright-hand side is a steering angle component calculated by feed-backcontrol with the target value of the lateral difference Dy being set tozero. The fourth term on the right-hand side is a steering anglecomponent that acts in a feed-back manner so that an integral value ΣDyof the lateral difference Dy is decreased. That is, the fourth term onthe right-hand side is a steering angle component calculated byfeed-back control with the target value of the integral value ΣDy beingset to zero.

The target steering angle θlta* is set as the steering angle of the leftdirection, for example, when the lane center line CL is curved in theleft direction, when the own vehicle is laterally shifted in the rightdirection with respect to the lane center line CL, and when the ownvehicle is facing the right direction with respect to the lane centerline CL. Further, the target steering angle θlta* is set as the steeringangle of the right direction when the lane center line CL is curved inthe right direction, when the own vehicle is laterally shifted in theleft direction with respect to the lane center line CL, and when the ownvehicle is facing the left direction with respect to the lane centerline CL. Therefore, the driving support ECU 10 calculates Expression (1)with use of signs corresponding to the right/left directions.

The driving support ECU 10 outputs, to the EPS·ECU 20, a command signalrepresenting the target steering angle θlta* that is the calculationresult. The EPS·ECU 20 controls the drive of the steering motor 22 sothat the steering angle follows the target steering angle θlta*. In thisembodiment, the driving support ECU 10 outputs the command signalrepresenting the target steering angle θlta* to the EPS·ECU 20, but thedriving support ECU 10 may calculate a target torque for obtaining thetarget steering angle θlta*, and output, to the EPS·ECU 20, a commandsignal representing the target torque that is the calculation result.

The driving support ECU 10 issues a lane departing alert by, forexample, sounding the buzzer 13 when the own vehicle has a probabilityof moving outside the lane while departing from the lane.

The above is the outline of the LTA.

<Adaptive Cruise Control (ACC)>

The adaptive cruise control refers to the following control. When apreceding vehicle traveling ahead of the own vehicle is present, the ownvehicle is caused to follow the preceding vehicle while maintaining aninter-vehicle distance between the preceding vehicle and the own vehicleto a predetermined distance based on the surrounding information. Whenthere is no preceding vehicle, the own vehicle is caused to travel at aconstant setting vehicle speed. In the following, the adaptive cruisecontrol is called “ACC”. The ACC itself is widely known (e.g., refer toJapanese Patent Application Laid-open No. 2014-148293, Japanese PatentApplication Laid-open No. 2006-315491, Japanese Patent No. 4172434, andJapanese Patent No. 4929777). Thus, a brief description is now given ofthe ACC.

The driving support ECU 10 is configured to carry out the ACC when theACC is requested by the operation on the setting operation unit 14. Thatis, the driving support ECU 10 is configured to select a followingsubject vehicle based on the surrounding information acquired from thesurrounding sensors 11 when the ACC is requested. For example, thedriving support ECU 10 determines whether or not another vehicle existsin a following subject vehicle area defined in advance.

When another vehicle is present in the following subject vehicle areafor a time equal to or more than a predetermined time, the drivingsupport ECU 10 selects that another vehicle as the following subjectvehicle, and sets a target acceleration so that the own vehicle followsthe following subject vehicle while keeping a predeterminedinter-vehicle distance between the own vehicle and the following subjectvehicle. When another vehicle is not present in the following subjectvehicle area, the driving support ECU 10 sets the target accelerationbased on the set vehicle speed and the detected speed (vehicle speeddetected by the vehicle speed sensor) so that the speed of the ownvehicle matches the set vehicle speed.

The driving support ECU 10 uses the engine ECU 50 to control the engineactuator 51, and, depending on necessity, uses the brake ECU 60 tocontrol the brake actuator 61 so that the acceleration of the ownvehicle matches the target acceleration.

When an accelerator operation is performed by the driver during ACC, theaccelerator operation is prioritized, and automatic deceleration controlfor keeping the inter-vehicle distance between the preceding vehicle andthe own vehicle is not performed.

The above is the outline of the ACC.

<Lane Change Assist Control (LCA)>

The lane change assist control refers to the following control. Afterthe surrounding of the own vehicle is monitored and it is determinedthat the own vehicle can safely change the lane, a steering torque isapplied to the steering mechanism so that the own vehicle is moved fromthe lane on which the own vehicle is currently traveling to the adjacentlane while the surrounding of the own vehicle is monitored. Thus, thedriver's steering operation (lane change operation) is assisted.Therefore, with the lane change assist control, the lane on which theown vehicle travels can be changed without the driver's steeringoperation (steering wheel operation). In the following, the lane changeassist control is called “LCA”.

Similarly to the LTA, the LCA is control of a lateral position of theown vehicle with respect to the lane, and is executed in place of theLTA when the lane change assist request is accepted while the LTA andthe ACC are executed. In the following, the LTA, the LCA, original lanereturn control described later, first yaw angle return control describedlater, and second yaw angle return control described later arecollectively referred to as “steering assist control”, and the state ofthe steering assist control is called “steering assist control state”.

The steering assist device executes control for assisting the steeringoperation by the driver. Therefore, when steering assist control (LTA,LCA, original lane return control, first yaw angle return control, andsecond yaw angle return control) is to be executed, the driving supportECU 10 generates a steering torque for steering assist control so thatthe steering wheel operation by the driver is prioritized. As a result,the driver can cause the own vehicle to move in an intended directionbased on the steering wheel operation by the driver even when thesteering assist control is executed.

FIG. 5 is a flowchart for illustrating a steering assist control routineexecuted by the driving support ECU 10. The steering assist controlroutine is executed when an LTA execution permission condition isestablished. The LTA execution permission condition may be, for example,the fact that execution of the LTA is selected by the setting operationunit 14, the fact that ACC is being executed, and/or the fact that whitelines can be recognized by the camera sensor 12.

In Step 11, when the steering assist control routine is started, thedriving support ECU 10 sets the steering assist control state to an LTAON state. The LTA ON state represents the control state in which the LTAis to be executed.

Next, in Step S12, the driving support ECU 10 determines whether or notan LCA start condition is established.

The LCA start condition is established when, for example, all of thefollowing conditions are established.

1. A lane change assist request operation (lane change assist requestsignal) is detected.

2. The execution of the LCA is selected by the setting operation unit14.

3. The camera sensor 12 recognizes a relative position of the ownvehicle with respect to the lane in the lane width direction and thewhite line present in the turn signal operation direction (the whiteline serving as a boundary between the original lane and the targetlane) is a broken line.

4. The result of determining whether or not the LCA can be executed dueto the monitoring of the surrounding is YES (object (another vehicle orthe like) that becomes an obstacle during the lane change is notdetected based on the surrounding information acquired from thesurrounding sensors 11, and it is determined that the own vehicle cansafely change the lane).

5. The road is a road for exclusive use by automobiles (road typeinformation acquired from the navigation ECU 70 indicates a road forexclusive use by automobiles).

6. The vehicle speed of the own vehicle is within an LCA permittedvehicle speed range.

For example, the condition 4 is established when the inter-vehicledistance between the own vehicle and another vehicle after the lanechange is estimated to be appropriately secured based on the relativespeed between the own vehicle and another vehicle traveling on thetarget lane.

Noted that, when, for example, the camera sensor 12 recognizes the whitelines of left and right sides of the lane on which the own vehicle istraveling simultaneously, the camera sensor 12 can recognize therelative position of the own vehicle with respect to the lane in thelane width direction.

Further, when the camera sensor 12 recognizes the lane width of eachlane of the road on which the own vehicle is traveling and the camerasensor 12 recognizes at least one of the white lines, the camera sensor12 can recognize the relative position of the own vehicle with respectto the lane in the lane width direction. On the other hand, when thecamera sensor 12 cannot (fails to) recognize both the pair of whitelines each of which defines each of the left and right sides of the laneon which the own vehicle is traveling, the camera sensor 12 cannotrecognize the relative position of the own vehicle with respect to thelane in the lane width direction. When the camera sensor 12 recognizesthe white line(s) unclearly (for example, when the white lines are faint(blur)), this state is treated as a state in which “the camera sensor 12cannot (fails to) recognize the white line(s).” in this embodiment.

The LCA start conditions is not required to include the above-mentionedconditions, and can be set as appropriate.

When it is determined that the LCA start condition is not established,the driving support ECU 10 returns the processing to Step S11, andcontinues to execute the LTA.

When the LCA start condition is established while the LTA is beingexecuted (Step S12: Yes), the driving support ECU 10 starts the LCA inStep S13. When the LCA is started, the driving support ECU 10 transmitsan LCA execution display command to the meter ECU 30. As a result, LCAexecution state is displayed on the display unit 31.

FIG. 8 is a diagram for illustrating an example of a screen 31 a(referred to as LTA screen 31 a) displayed on the display unit 31 duringexecution of the LTA and a screen 31 b (referred to as an LCA screen 31b) displayed during execution of the LCA. An image in which the ownvehicle is traveling between the right and left white lines is displayedon the LTA screen 31 a and on the LCA screen 31 b. On the LTA screen 31a, virtual walls GW are displayed on an outer side of each of right andleft white line displays GWL. The driver can recognize from those wallsGW that the own vehicle is being controlled so as to travel within thelane.

On the other hand, on the LCA screen 31 b, the display of the virtualwalls GW is omitted, and in place of that display, an LCA trajectory Zis displayed. The driving support ECU 10 switches the screen to bedisplayed on the display unit 31 between the LTA screen 31 a and the LCAscreen 31 b depending on the steering assist control state. As a result,the driver can easily discriminate whether or not the execution state ofthe steering assist control is the LTA or the LCA.

The LCA is only control for assisting the steering operation by thedriver for changing lanes. The driver still has a duty to monitor his orher surroundings. Therefore, a message GM, namely, “Please check yoursurroundings yourself”, for causing the driver to monitor his or hersurroundings is displayed on the LCA screen 31 b.

When the LCA starts, first, in Step S13 of the routine illustrated inFIG. 5, the driving support ECU 10 calculates the target trajectory. TheLCA target trajectory is now described.

When executing the LCA, the driving support ECU 10 calculates a targettrajectory function for deciding the target trajectory of the ownvehicle. The target trajectory is a trajectory along which the ownvehicle is moved for a target lane change time period from the lane(called “original lane”) on which the own vehicle is currently travelingto the center position in the width direction (called “final targetlateral position”) of the lane (called “target lane”) present in thelane change assist request direction, which is adjacent to the originallane. The target trajectory has, for example, a shape as illustrated inFIG. 9.

The target trajectory function is, as described later, a function forcalculating a target value for the lateral position (target lateralposition) of the own vehicle corresponding to an elapsed time t based onthe lane center line CL of the original lane as a reference, and uses anelapsed time t from an LCA start time (point at which LCA startcondition is established) as a variable. The lateral position of the ownvehicle represents the center of gravity of the own vehicle in the lanewidth direction (also referred to as lateral direction) based on thelane center line CL as a reference.

The target lane change time is variably set in proportion to thedistance (hereinafter referred to as “required lateral distance”) thatthe own vehicle is to move in the lateral direction from an initialposition, which is the LCA start position (lateral position of the ownvehicle at the LCA start point) until a final target lateral position.For example, when the lane width is 3.5 m as in the case of generalroads, the target lane change time period is set to, for example, 8.0seconds. This example corresponds to a case in which the own vehicle ispositioned on the lane center line CL of the original lane at the startof the LCA. The target lane change time is adjusted in proportion to thewidth of the lane. Therefore, the target lane change time is set to alarger value as the lane is wider, and conversely, to a smaller value asthe lane is narrower.

Further, when the lateral-direction position of the own vehicle at thestart of the LCA is shifted to the lane change side with respect to thelane center line CL of the original lane, the target lane change timeperiod is set to be decreased as the shift amount (lateral differenceDy) is increased. On the other hand, when the lateral-direction positionof the own vehicle at the start of the LCA is shifted to the oppositeside of the lane change side with respect to the lane center line CL ofthe original lane, the target lane change time period is set to beincreased as the shift amount (lateral difference Dy) is increased. Forexample, when the shift amount is 0.5 m, the increase/decreaseadjustment amount of the target lane change time period may be 1.14seconds (=8.0×0.5/3.5). The value for setting the target lane changetime shown here is merely one example, and an arbitrarily set value canbe used.

In this embodiment, a target lateral position y is calculated based on atarget trajectory function y(t) represented by Expression (2). Thelateral position function y(t) is a fifth-order function in which theelapsed time t is a variable.

y(t)=c ₀ +c ₁ ·t+c ₂ ·t ² +c ₃ ·t ³ +c ₄ t ⁴ +c ₅ ·t ⁵  (2)

This target trajectory function y(t) is set to a function such that theown vehicle is smoothly moved to a final target lateral position.

In Expression (2), coefficients c₀, c₁, c₂, c₃, c₄, and c₅ aredetermined based on the state of the own vehicle when the LCA starts(initial lateral state amount) and a target state (final target lateralstate amount) of the own vehicle when the LCA is complete.

For example, as illustrated in FIG. 10, the target trajectory functiony(t) is a function for calculating a target lateral position y(t) of anown vehicle C corresponding to an elapsed time t (also sometimesreferred to as current time t) from the LCA start point (calculationpoint of the target trajectory), based on the lane center line CL of thelane (original lane) on which the own vehicle C is traveling at thecurrent time point. In FIG. 10, the lane is formed in a straight line,but when the lane is formed in a curve, the target trajectory functiony(t) is a function for calculating, based on the lane center line CLformed in a curve, the target lateral position of the own vehiclerelative to the lane center line CL.

The driving support ECU 10 sets target trajectory calculation parametersin the following manner in order to determine the coefficients c₀, c₁,c₂, c₃, c₄, and c₅ of the target trajectory function y(t). The targettrajectory calculation parameters include the following seven (P1 to P7)parameters.

P1. Lateral position of the own vehicle relative to the lane center lineof the original lane when the LCA starts (referred to as initial lateralposition).

P2. Speed of the own vehicle in the lateral direction when the LCAstarts (referred to as initial lateral speed).

P3. Acceleration of the own vehicle in the lateral direction when theLCA starts (referred to as initial lateral acceleration).

P4. Target lateral position (referred to as final target lateralposition) of the own vehicle relative to the lane center line of theoriginal lane when the LCA is complete (referred to as LCA completionpoint).

P5. Target speed of the own vehicle in the lateral direction when theLCA is complete (referred to as final target lateral speed).

P6. Target acceleration of the own vehicle in the lateral direction whenthe LCA is complete (referred to as final target lateral acceleration).

P7. Target time, which is a target value of the time for executing theLCA (time from the start of the LCA until the LCA completion point)(referred to as target lane change time).

As described above, the lateral direction is the lane width direction.Therefore, the lateral speed represents the speed of the own vehicle inthe width direction of the lane, and the lateral acceleration representsthe acceleration of the own vehicle in the width direction of the lane.

The processing for setting those seven target trajectory calculationparameters is referred to as initialization processing. In thisinitialization processing, the target trajectory calculation parametersare set in the following manner. Specifically, the initial lateralposition is set to a value equivalent to the lateral deviation Dydetected by the camera sensor 12 when the LCA starts. The initiallateral speed is set to a value (v·sin(θy)) obtained by multiplying thesine value sin(θy) of the yaw angle θy detected by the camera sensor 12by a vehicle speed v detected by the vehicle speed sensor when the LCAstarts. The initial lateral acceleration is set to a value (v·γ)obtained by multiplying the vehicle speed v by a yaw rate γ (rad/s)detected by the yaw rate sensor when the LCA starts. However, theinitial lateral acceleration can also be set to a derivative value ofthe initial lateral speed. The initial lateral position, the initiallateral speed, and the initial lateral acceleration are collectivelyreferred to as the initial lateral state amount.

The driving support ECU 10 in this embodiment considers the lane widthof the target lane to be the same as the lane width of the original lanedetected by the camera sensor 12. Therefore, the final target lateralposition is set to the same value as the lane width of the original lane(final target lateral position=lane width of original lane). The drivingsupport ECU 10 sets the value of the final target lateral speed and thevalue of the final target acceleration to zero. The final target lateralposition, the final target lateral speed, and the final target lateralacceleration are collectively referred to as the final target lateralstate amount.

The target lane change time is, as described above, calculated based onthe lane width (the lane width of the original lane may be used) and thelateral-direction shift amount of the own vehicle when the LCA starts.

For example, a target lane change time t_(len) is calculated byExpression (3).

t _(len) =D _(ini) A  (3)

In Expression (3), D_(ini) is the required distance that the own vehicleis to be moved in the lateral direction from the LCA start position(initial lateral position) to an LCA completion position (final targetlateral position). Therefore, when the own vehicle is positioned on thelane center line CL of the original lane at the start time of the LCA,D_(ini) is set to a value equivalent to the lane width, and when the ownvehicle is shifted from the lane center line CL of the original lane,D_(ini) is a value obtained by adding or subtracting that shift amountto/from the lane width. Symbol A is a constant (referred to as a targettime setting constant) representing the target time to be taken in orderto move the own vehicle in the lateral direction by a unit distance. Forexample, symbol A is set to (8 sec/3.5 m=2.29 sec/m). In this example,when the required distance D_(ini) that the own vehicle is to be movedin the lateral is 3.5 m, the target lane change time t_(len) is set to 8seconds.

The target time setting constant A is not limited to the above-mentionedvalue, and can be set arbitrarily. For example, the target time settingconstant A may be selected from among a plurality of options inaccordance with a preference of the driver by using the settingoperation unit 14. The target lane change time can also be a fixedvalue.

The driving support ECU 10 calculates the coefficients c₀, c₁, c₂, c₃,c₄, and c₅ of the target trajectory function y(t) represented byExpression (2) based on the initial lateral state amount, the finaltarget lateral state amount, and the target lane change time, all ofwhich are determined by the initialization processing of the targettrajectory calculation parameters, and confirms the target trajectoryfunction y(t).

From the target trajectory function y(t) represented by Expression (2),a lateral speed y′ (t) of the own vehicle can be represented byExpression (4), and a lateral acceleration y″ (t) of the own vehicle canbe represented by Expression (5).

y′(t)=c ₁+2c ₂ ·t+3c ₃ ·t ²+4c ₄ ·t ³+5c ₅ ·t ⁴  (4)

y″(t)=2c ₂+6c ₃ ·t+12c ₄ ·t ²+20c ₅ ·t ³  (5)

In Expressions (4) and (5), when the initial lateral position isrepresented as y₀, the initial lateral speed as vy₀, the initial lateralacceleration as ay₀, the final target lateral position as y₁, the finaltarget lateral speed as vy₁, the final target lateral acceleration asay₁, and the lane width of the original lane as W, based on theabove-mentioned target trajectory calculation parameters, the followingrelational Expressions are obtained.

y(0)=c ₀ =y ₀  (6)

y′(0)=c ₁ =vy ₀  (7)

y″(0)=2c ₂ =ay ₀  (8)

y(t _(len))=c ₀ +c ₁ ·t _(len) +c ₂ ·t _(len) ² +c ₃ ·t _(len) ³ +c ₄ ·t_(len) ⁴ +c ₅ t _(len) ⁵ =y ₁ =W  (9)

y′(t _(len))=c ₁+2c ₂ ·t _(len)+3c ₃ ·t _(len) ²+4c ₄ ·t _(len) ³+5c ₅·t _(len) ⁴ =vy ₁=0  (10)

y″(t _(len))=2c ₂+6c ₃ ·t _(len) ²+12c ₄ ·t _(len) ²+20c ₅ ·t _(len) ³=ay ₁=0  (11)

Therefore, the values of the coefficients c₀, c₁, c₂, c₃, c₄, and c₅ ofthe target trajectory function y(t) can be calculated from the sixrelational Expressions (6) to (11). The target trajectory function y(t)is calculated by substituting the values of the calculated coefficientsc₀, c₁, c₂, c₃, c₄, and c₅ into Expression (2). The driving support ECU10 stores and maintains the target trajectory function y(t) until theLCA is ended. At the same time as calculating the target trajectoryfunction y(t), the driving support ECU 10 also activates a clock timer(initial value: zero), and starts to count up the elapsed time t fromthe start of the LCA.

When the target trajectory function has been calculated in this way, inthe following Step S14, the driving support ECU 10 performs steeringcontrol based on the target trajectory function. This steering controlis now specifically described.

First, the driving support ECU 10 calculates the target lateral stateamount of the own vehicle at the current time point. The target lateralstate amount includes the target lateral position, which is the targetvalue for the lateral position of the own vehicle in the lane widthdirection, the target lateral speed, which is the target value for thespeed (lateral speed) of the own vehicle in the lane width direction,and the target lateral acceleration, which is the target value for theacceleration (lateral acceleration) of the own vehicle in the lane widthdirection. The lateral speed and the lateral acceleration arecollectively referred to as a lateral movement state amount, and thetarget lateral speed and the target lateral acceleration are sometimescollectively referred to as a target lateral movement state amount.

In this case, the driving support ECU 10 calculates, based on the targettrajectory function y(t) confirmed in Step S13 and the current time t,the target lateral position, the target lateral speed, and the targetlateral acceleration at the current time point. The current time t isthe time that has elapsed since the target trajectory function y(t) isconfirmed in Step S13, and is the same as the elapsed time from thestart of the LCA. In Step S13, when the target trajectory function y(t)has been calculated, the driving support ECU 10 resets the clock timerand starts to count up the elapsed time t (=current time t) from thestart of the LCA. The target lateral position is calculated bysubstituting the current time t into the target trajectory functiony(t). The target lateral speed is calculated by substituting the currenttime t into a function y′ (t) obtained by first-order differentiation ofthe target trajectory function y(t), and the target lateral accelerationis calculated by substituting the current time t into a function y″ (t)obtained by second-order differentiation of the target trajectoryfunction y(t). The driving support ECU 10 reads the elapsed time tmeasured by the timer, and based on this measured time t and theabove-mentioned functions, calculates the target lateral state amount.

In the following description, the target lateral position at the currenttime is represented as y*, the target lateral speed at the current timeis represented as vy*, and the target lateral acceleration at thecurrent time is represented as ay*.

Next, the driving support ECU 10 calculates a target yaw state amount,which is a target value relating to movement for changing the directionof the own vehicle. The target yaw state amount represents, at thecurrent time point, a target yaw angle θy* of the own vehicle, a targetyaw rate γ* of the own vehicle, and a target curvature Cu*. The targetcurvature Cu* is the curvature of the trajectory for changing the laneof the own vehicle, namely, the curvature of the curve componentrelating to lane change that does not include the curvature of the curveof the lane.

The driving support ECU 10 reads the vehicle speed v at the current timepoint (current vehicle speed detected by the vehicle speed sensor), andbased on this vehicle speed v, a target lateral speed vy*, and a targetlateral acceleration ay*, calculates the target yaw angle θy*, thetarget yaw rate γ*, and the target curvature Cu* at the current timepoint by using following Expressions (12), (13), and (14).

θy*=sin⁻¹(vy*/v)  (12)

γ*=ay*/v  (13)

Cu*=ay*/v ²  (14)

Specifically, the target yaw angle θy* is calculated by substituting avalue obtained by dividing the target lateral speed vy* by the vehiclespeed v into an arcsine function. The target yaw rate γ* is calculatedby dividing the target lateral acceleration ay* by the vehicle speed v.The target curvature Cu* is calculated by dividing the target lateralacceleration ay* by the square of the vehicle speed v.

Next, the driving support ECU 10 calculates the target control amount ofthe LCA. In this embodiment, a target steering angle θ_(lca)* iscalculated as the target control amount. The target steering angleθ_(lca)* is calculated by Expression (15) based on the target lateralposition y*, the target yaw angle θy*, the target yaw rate γ*, thetarget curvature Cu*, and the curvature Cu calculated in the mannerdescribed above.

θ_(lca) *=K _(lca)1·(Cu*+Cu)+K _(lca)2·(θy*−θy)+K _(lca)3(γ*−γ)+K_(lca)4·(γ*−γ)+K _(lca)5·Σ(y*−y)  (15)

In Expression (15), K_(lca)1, K_(lca)2, K_(lca)3, K_(lca)4, and K_(lca)5each represent a control gain. The symbol Cu represents the curvature atthe current time point (during calculation) detected by the camerasensor 12. The symbol y represents the lateral position at the currenttime point (during calculation) detected by the camera sensor 12,namely, γ corresponds to Dy. The symbol By represents the yaw angle atthe current time point (during calculation) detected by the camerasensor 12. The symbol γ represents the yaw rate of the own vehicle atthe current time point detected by the yaw rate sensor. The derivativevalue of the yaw angle θy can also be used as γ.

The first term on the right-hand side is a feed-forward control amountdetermined in accordance with a value obtained by adding the targetcurvature Cu* and the curvature Cu (curve of the lane). K_(lca)1·Cu* isthe feed-forward control amount for performing lane change. K_(lca)1·Cuis the feed-forward control amount for causing the own vehicle to travelalong the curve of the lane. Therefore, the control amount representedby the first term on the right-hand side is basically set to a valuecapable of causing the own vehicle to travel along a target travel pathwhen the steering angle is controlled by that control amount. In thiscase, the control gain K_(lca)1 is set to a value that depends on thevehicle speed v. For example, the control gain K_(lca)1 may be set likein Expression (16) in accordance with a wheel base L and a stabilityfactor Ksf (fixed value determined for each vehicle). In this case, K isa fixed control gain.

K _(lca)1=K·L·(1+Ksf·v ²)  (16)

The second to fifth terms on the right-hand side in Expression (15) eachrepresent a feedback control amount. The second term on the right-handside represents a steering angle component for providing feedback so asto reduce a deviation between the target yaw angle θy* and an actual yawangle θy. The third term on the right-hand side represents a steeringangle component for providing feedback so as to reduce a deviationbetween the target lateral position y* and an actual lateral position y.The fourth term on the right-hand side represents a steering anglecomponent for providing feedback so as to reduce a deviation between thetarget yaw rate γ* and an actual yaw rate γ. The fifth term on theright-hand side represents a steering angle component for providingfeedback so as to reduce an integral value Σ(y*−Y) of a deviationbetween the target lateral position y* and the actual lateral positiony.

The target steering angle θ_(lca)* is not limited to an angle calculatedbased on the above-mentioned five steering components. The targetsteering angle θ_(lca)* can be calculated using only arbitrary steeringcomponents from among those five, or can also be calculated by, forexample, additionally using other steering components. For example,regarding the feedback control amount relating to yaw movement, any oneof a deviation in the yaw angle and a deviation in the yaw rate can beused. Further, the feedback control amount obtained using the integralvalue Σ(y*−Y) of the deviation between the target lateral position y*and the actual lateral position y can be omitted.

Further, in Step S13, the driving support ECU 10 performs a guardprocess on the calculated target steering angle θlca*.

That is, the driving support ECU 10 defines (calculates) an upper limitvalue of an absolute value of curvature (target curvature) of the targettrajectory of the own vehicle which is defined by the target trajectoryfunction y(t) and an upper limit value of an absolute value of thechange in gradient (change rate per unit time) of curvature (targetcurvature). The curvature (the target curvature) corresponds to thesteering angle (target steering angle) of the own vehicle and the changein gradient of the curvature corresponds to the steering angularvelocity (target steering angular velocity) of the own vehicle.

Here, the upper limit value of the absolute value of the curvature ofthe target trajectory (the steering angle) during the LCA is limited by(the absolute value of) a predetermined steering angle guard at lanechange time, and the upper limit value of the absolute value of thechange in gradient of the curvature (the steering angular velocity) islimited by (the absolute value of) a predetermined steering angularvelocity guard at lane change time.

Therefore, when the absolute value of the target curvature (targetsteering angle) corresponding to the target steering angle θlca* islarger than the absolute value of the steering angle guard at lanechange time, the driving support ECU 10 corrects (recalculates) thetarget steering angle θlca* so that the target curvature (targetsteering angle) becomes the same as the value of the steering angleguard at lane change time. Similarly, when the absolute value of thechange in gradient of the target curvature (target steering angularvelocity) corresponding to the target steering angle θlca* is largerthan the absolute value of the steering angular velocity guard at lanechange time, the driving support ECU 10 corrects (recalculates) thetarget steering angle θlca* so that the change in gradient of the targetcurvature (target steering angular velocity) becomes the same as thevalue of the steering angular velocity guard at lane change time.

On the other hand, when the absolute value of the target curvature(target steering angle) corresponding to the target steering angle θlca*is equal to or smaller than the absolute value of the steering angleguard at lane change time and the absolute value of the change ingradient of the target curvature (target steering angular velocity)corresponding to the target steering angle θlca* is equal to or smallerthan the absolute value of the steering angular velocity guard at lanechange time, the target steering angle θlca* recalculated by the drivingsupport ECU 10 is the same as the target steering angle θlca* beforerecalculation. That is, in this case, the driving support ECU 10 doesnot correct the target steering angle θlca*.

When calculating or correcting the target control amount in this way,the driving support ECU 10 transmits the steering command representingthe target control amount to the EPS·ECU 20. In this embodiment, thedriving support ECU 10 calculates the target steering angle θlca* as thetarget control amount, but the driving support ECU 10 may calculate atarget torque for obtaining the target steering angle θlca*, andtransmit a steering command representing that target torque to theEPS·ECU 20.

The processing described above is the processing of Step S14.

When receiving a steering command from the driving support ECU 10 viathe CAN 100, the EPS·ECU 20 controls the drive of the steering motor 22so that the steering angle follows the target steering angle θlca*calculated (or corrected) by the driving support ECU 10.

Next, in Step S15, the driving support ECU 10 determines whether or nota predetermined LCA interruption condition is established.

The LCA interruption condition is established when any one of thefollowing first interruption condition to third interruption conditionis established.

First interruption condition: The estimated time period (a collisiontime TTC) from the current time until the own vehicle collides withanother vehicle is less than the threshold TTCth when the LCA isexecuted.Second interruption condition: The camera sensor 12 cannot (fails to)recognize the relative position of the own vehicle in the lane widthdirection with respect to the lane.Third interruption condition: The steering torque which is input to thesteering wheel and is detected by the steering torque sensor exceeds apredetermined value.

The driving support ECU 10 determines whether or not the firstinterruption condition is established in Step S15. That is, the drivingsupport ECU10 calculates the estimated time period (the collision timeTTC: Time to collision) from the current time until the own vehiclecollides with another vehicle(s) based on the relative speed withrespect to the “another vehicle(s) existing on the original lane and/orthe target lane adjacent to the original lane” and the distance betweenthe own vehicle and another vehicle(s). Then, the driving support ECU 10determines whether or not the collision time TTC is equal to or morethan a threshold TTCth. When the collision time TTC is equal to or morethan the threshold TTCth, the first interruption condition is notestablished. On the other hand, when the collision time TTC is less thanthe threshold TTCth, the first interruption condition is established.Further, the driving support ECU 10 outputs the surrounding monitoringresult. When the collision time TTC is equal to or more than thethreshold TTCth, the surrounding monitoring result is “there is noapproaching vehicle”. When the collision time TTC is less than thethreshold TTCth, the surrounding monitoring result is “there is anapproaching vehicle”.

Furthermore, the driving support ECU 10 determines whether or not thesecond interruption condition is established in Step S15. That is, thedriving support ECU 10 determines whether or not the camera sensor 12can recognize the relative position of the own vehicle in the lane widthdirection with respect to the lane at the current time. For example,when the camera sensor 12 simultaneously can recognize a pair of whitelines WL defining left and right side edge portions of the lane on whichthe own vehicle is traveling respectively, the second interruptioncondition is not established. On the other hand, when the camera sensor12 simultaneously cannot (fails to) recognize the pair of white lines WLdefining the left and right side edge portions of the lane on which theown vehicle is traveling respectively, the second interruption conditionis established.

Further, in Step S15, the driving support ECU 10 determines whether ornot the third interruption condition is established. That is, thedriving support ECU 10 determines whether or not the steering torque ofthe steering wheel which is detected by the steering torque sensorexceeds the predetermined value at the current time. When the steeringtorque of the steering wheel does not exceed the predetermined value,the third interruption condition is not established. On the other hand,when the steering torque of the steering wheel exceeds the predeterminedvalue, the third interruption condition is established.

When determining No in Step S15, the driving support ECU 10 proceeds toStep S16 to determine whether or not an LCA completion condition isestablished. In this embodiment, the LCA completion condition isestablished when the lateral position y of the own vehicle has reachedthe final target lateral position y*. When the LCA completion conditionis not established, the driving support ECU 10 returns the processing toStep S14, and repeats the processing of Steps S14 to S16 at apredetermined calculation cycle. In this way, the LCA is continued.

During execution of the LCA, target lateral state amounts (y*, vy*, anday*) that depend on the elapsed time t are calculated. In addition,based on the calculated target lateral state amounts (y*, vy*, and ay*)and the vehicle speed v, target yaw state amounts (θy*, y*, and Cu*) arecalculated, and based on the calculated target yaw state amounts (θy*,y*, and Cu*), the target control amount (θ_(lca)*) is calculated. Eachtime the target control amount (θ_(lca)*) is calculated, a steeringcommand representing the target control amount (θ_(lca)*) is transmittedto the EPS·ECU 20. In this way, the own vehicle travels along the targettrajectory.

When the travel position of the own vehicle switches during execution ofthe LCA from the original lane to the target lane, the lane-relatedvehicle information (Cu, Dy, and By) supplied to the driving support ECU10 from the camera sensor 12 switches from lane-related vehicleinformation on the original lane to lane-related vehicle information onthe target lane. As a result, it is not possible to use the targettrajectory function y(t) initially calculated when the LCA started asis. When the lane on which the own vehicle is positioned switches, thesign of the lateral deviation Dy reverses. Therefore, when the drivingsupport ECU 10 detects that the sign (plus or minus) of the lateraldeviation Dy output by the camera sensor 12 has switched, the drivingsupport ECU 10 offsets the target trajectory function y(t) by the lanewidth W of the original lane. This enables the target trajectoryfunction y(t) calculated using the lane center line CL of the originallane as an origin to be converted into a target trajectory function y(t)in which the lane center line CL of the target lane is the origin.

When it is determined in Step S16 that the LCA completion condition isestablished, in Step S17, the driving support ECU 10 sets the steeringassist control state to an LTA ON state, that is, completes the LCA andrestarts the LTA. As a result, steering is controlled so that the ownvehicle travels along the lane center line CL of the target lane. Whenthe steering assist control state is set to an LTA ON state in Step S17,the driving support ECU 10 returns the processing to Step S11, andcontinues the steering assist control routine described above as is.

When the LCA is complete and the steering assist control state is set toan LTA ON state, the screen displayed on the display unit 31 is switchedto the LTA screen 31 a from the LCA screen 31 b, as illustrated in FIG.8.

During the period from the start of the LCA until the end of thesteering assist control routine, the driving support ECU 10 transmits tothe meter ECU 30 a flashing command of the turn signal 32 correspondingto the turn signal operation direction. From before the LCA is started,the turn signal 32 flashes based on a flashing command transmitted fromthe steering ECU 40 due to an operation of the turn signal lever 41 tothe first stroke position P1L (P1R). The meter ECU 30 continues theflashing of the turn signal 32 during the period that the flashingcommand is transmitted from the driving support ECU 10, even when theflashing command transmitted from the steering ECU 40 is stopped.

Next, a case where the driving support ECU 10 determines Yes in Step S15will be described. In this case, the driving support ECU 10 advances theprocessing to Step S60. FIG. 6 is a flowchart showing the processing ofStep S60 (subroutine A).

First, in Step S61, the driving support ECU 10 determines whether or notthe third interruption condition is established.

When determining Yes in Step S61, the drive support ECU 10 proceeds toStep S62.

In this case, since the driver is considered to wish to steer thesteering wheel by himself/herself and interrupt the LCA, the drivingsupport ECU 10 immediately terminates the LCA.

Further, the driving support ECU 10 sets an original lane return flag to“0”. The initial value of the original lane return flag is “0”.

Upon completion of the processing of Step S62, the driving support ECU10 temporarily ends the processing of the subroutine A.

On the other hand, when determining No in Step S61, the driving supportECU 10 proceeds to Step S63.

During the execution of the LCA, as shown in FIG. 13, there is a casewhere another vehicle C2, which is traveling on the target lane and ispositioned behind the own vehicle C1, rapidly approaches the own vehicleC1 at an unexpected relative speed. Further, there is a case whereanother vehicle C3, which traveled on a lane adjacent to the target lane(a lane separated from the original lane by two lanes) enters the targetlane and abnormally approaches the own vehicle C1. Further, there is acase where another vehicle, which was positioned in the blind spot rangeof the surrounding sensors 11, abnormally approaches the own vehicle.

In this way, for example, when the collision time TTC becomes less thanthe threshold TTCth in the case where another vehicle C3 is abnormallyapproaching, the driving support ECU 10 determines Yes in Step S63 toproceed to Step S64. Then, in Step S64, the driving support ECU 10executes processing for supporting avoidance of collision with anothervehicle by issuing an alarm to the driver and changing the movement ofthe own vehicle in a short time so that the own vehicle does not move tothe center of the target lane in the width direction.

In Step S64, the driving support ECU 10 sets the steering assist controlstate to a first yaw angle return control state. When the steeringassist control state is set to the first yaw angle return control state,the LCA is terminated.

Further, in Step S64, the driving support ECU 10 calculates a first yawangle return target trajectory (see FIG. 13) for returning the yaw angleof the own vehicle to a state (the yaw angle) immediately before thestart of the LCA.

Here, the first yaw angle return target trajectory is described. Thefirst yaw angle return target trajectory is a target trajectory formaking the yaw angle of the own vehicle zero as brief a period aspossible so as not to cause a problem on running stability of thevehicle. In other words, the first yaw angle return target trajectory isa target trajectory for making the lateral speed of the own vehicle inthe lane change direction zero as brief a period as possible so as notto cause a problem on running stability of the vehicle. Immediatelybefore the start of the LCA, the LTA is being executed. Therefore, whenthe LCA is started, the yaw angle is estimated to be a value close tozero. Therefore, the driving support ECU 10 calculates the first yawangle return target trajectory for returning the yaw angle generated inthe LCA to the state (the yaw angle) at the lane change start time,which is the start time of the LCA, to cancel the target lateral speedvy* calculated from the target trajectory function of the LCA (to makethe target lateral speed vy* zero).

This target trajectory during the LCA represents the target lateralposition versus (corresponding to) the elapsed time from the lane changestart time. However, the first yaw angle return target trajectory isdefined by the target curvature versus (corresponding to) the elapsedtime from the point when an approaching vehicle is detected. The targetcontrol amount to be finally output to the EPS·ECU 20 is set to a valueobtained by multiplying a control gain (a coefficient for convertingcurvature into a steering angle, which can be the above-mentionedcontrol gain K_(lca)1) by a value obtained by adding together thistarget curvature and the curvature (curvature of the curve of the lane)detected by the camera sensor 12.

A method of returning the yaw angle to the state at the lane changestart time t0 along the first yaw angle return target trajectoryreferring to FIG. 11. The target control amount during the LCA isrepresented by the target steering angle θlca. This target steeringangle θlca* includes, as shown by Expression (15), a feed-forward term(Klca1−Cu*) calculated from the target curvature Cu*.

The change in the target curvature corresponds to a change in thesteering angle, and can be grasped as a change in the yaw angle.Therefore, when an approaching vehicle is detected, the yaw angle can bereturned to the state at the lane change start time t0 by calculatingthe integral value of the target curvature Cu* during the time periodfrom the lane change start time t0 being the start time of the LCA untilthe approaching vehicle is detected, reversing the sign of the controlamount corresponding to the integral value of the target curvature Cu*,and outputting the control amount to the EPS·ECU 20.

For example, a graph of FIG. 11 shows the case where an approachingvehicle is detected at a time t1 a. In other words, the graph of FIG. 11shows the case where the first interruption condition is established atthe time t1 a. When the approaching vehicle is detected at the time t1a, the integral value of the target curvature Cu* from the lane changestart time t0 at which the LCA starts to the time t1 a corresponds tothe surface area of the portion colored in gray in FIG. 11. Therefore,when the sign of the feed-forward control amount corresponding to thatsurface area is reversed (the left-right direction is reversed) and thefeed-forward control amount is issued as a command to the EPS·ECU 20,the yaw angle can be returned to the state at the lane change start timet0 at the point when output of the feed-forward control amount iscomplete. The value obtained by reversing the sign (plus or minus) of afirst integral value Int1, which is the integral value of the targetcurvature Cu* from the lane change start time t0 to the time t1 a, isreferred to as a first inverse integral value Intr1. This first inverseintegral value Intr1 corresponds to the surface area of a trapezoidalportion formed under the abscissa axis (time axis) between a time t2 a−1and a time t3 a in FIG. 11. The value obtained by adding this firstinverse integral value Intr1 to the first integral value Int1 of thetarget curvature Cu* from the lane change start time t0 to the time t1 ais zero.

When the approaching vehicle (another vehicle may abnormally approachthe own vehicle on the target lane) is detected at the time t1 a, asshown in FIG. 13, a part of the own vehicle is entering the target laneor the distance from the own vehicle to the target lane is short. Thus,this state is an emergency state. Therefore, the own vehicle is requiredto be made to be parallel to the formation direction of the lane byreturning the yaw angle to zero as brief a period as possible.Meanwhile, in the control system of the steering assist device, theupper limit of the magnitude of the vehicle's lateral acceleration(lateral acceleration which acts on the vehicle and is different fromlateral acceleration in the lane width direction) and the upper limit ofthe magnitude of change rate which is allowed to be used for changingthe lateral acceleration (the magnitude of change amount of the lateralacceleration per unit time) are set in advance.

Accordingly, as shown in the heavy line of FIG. 11, the driving supportECU 10 calculates a first target curvature Cuemergency 1* which is thetarget curvature at or after the time t1 a. This first target curvatureCuemergency 1* is defined by the outline (visible outline) of the firstintegral value Int1 (the trapezoidal shape) between the time t2 a−1 andthe time t3 a in FIG. 11. The first target curvature Cuemergency 1* iscalculated by using the maximum value (Cumax) and the maximum change ingradient (Cu′max). The maximum value (Cumax) is set to the upper limitof the lateral acceleration of the vehicle which is allowed in thecontrol system of the steering assist device. The maximum change ingradient (Cu′max) represents a change in gradient to increase the firsttarget curvature Cuemergency 1* to the maximum value (Cumax) and achange in gradient to decrease the first target curvature Cuemergency 1*to zero from the maximum value Cumax, and is set as the upper limitwhich is allowed in the control system of the steering assist device.For example, the maximum value Cumax is set to a value so that thelateral acceleration of the vehicle becomes 0.2 G (G: gravitationalacceleration). A lateral acceleration YG acting on the vehicle can becalculated by multiplying the square of the vehicle speed (v²) by acurvature (Cu) (YG=·v²·Cu). Therefore, based on this relationalexpression, the maximum value Cumax can be calculated. Noted that thesign of the maximum value Cumax and the sign of the maximum change ingradient Cu′max are determined by the sign of the first inverse integralvalue Intr1.

The driving support ECU 10 calculates the first target curvatureCuemergency 1* versus (corresponding to) the elapsed time t from thetime point (a time t1 a in FIG. 11) at which an approaching vehicle isdetected based on the value of the first inverse integral value Intr1,the maximum value Cumax of the target curvature, and the maximum changein gradient Cu ′max of the target curvature. In other words, the drivingsupport ECU 10 calculates the elapsed time (the upper base of thetrapezoidal shape) from a time t2 a−1 to a time t3 a and time ΔT (thelower base of the trapezoidal shape) in which the maximum value Cumax ismaintained based on the maximum value Cumax and the maximum change ingradient Cu ′max to determine the outer shape of the first inverseintegral value Intr1. Further, the driving support ECU 10 calculates thefirst target curvature Cuemergency 1* between the time t1 a and the timet2 a−1 by extending the inclined straight line which is the outer shapeof the first inverse integral value Intr1 between a time t2 a−2, atwhich the first target curvature Cuemergency 1* becomes the maximumvalue (Cumax), and the time t2 a−1. Hereinafter, the first targetcurvature Cuemergency 1* versus (corresponding to) the elapsed time tmay be referred to as a first target curvature function Cuemergency1*(t). The first target curvature function Cuemergency 1*(t) determinesthe target trajectory of the own vehicle. Therefore, the first targetcurvature function Cuemergency 1*(t) corresponds to the first yaw anglereturn target trajectory.

The first target curvature Cuemergency 1* at a predetermined time andthe target steering angle θlca* at this predetermined time correspond toeach other. Furthermore, the change in gradient of the first targetcurvature Cuemergency 1* at this predetermined time and the steeringangular velocity (that is, the target steering angular velocity) whichis the change amount of the steering angle per unit time at thispredetermined time correspond to each other. Therefore, the maximumvalue Cumax determines the maximum value of the target steering angleθlca* between the time t1 a and the time t3 a, and the maximum change ingradient Cu′max determines the maximum value of the target steeringangular velocity between the time t1 a and the time t3 a.

The first inverse integral value Intr1 can be calculated by integrating(adding up) target curvatures Cu* every time a target curvature Cu* iscalculated during execution of the LCA and reversing the sign of theintegral value. However, in this embodiment, the first inverse integralvalue Intr1 is calculated as follows.

The target curvature Cu* during the LCA can be represented like inExpression (19) by using the target lateral acceleration ay* and thevehicle speed v.

Cu*=ay*/v ²  (19)

Therefore, the value obtained by integrating this target curvature Cu*from the lane change start time t0 (elapsed time t=0) to the time t1 a(elapsed time t=t1 a) can be represented like in Expression (20) byusing the vehicle speed v and the target lateral speed vy*. Expression(20) is based on the assumption that the vehicle speed v can be assumedto be fixed during execution of the LCA.

$\begin{matrix}\begin{matrix}{{\int_{0}^{t\; 1}{{{Cu}^{*}(t)}{dt}}} = \lbrack \frac{{vy}^{*}(t)}{v^{2}} \rbrack_{0}^{t\; 1}} \\{= \frac{{vy}^{*}( {t\; 1} )}{v^{2}}}\end{matrix} & (20)\end{matrix}$

Therefore, the first inverse integral value Intr1 is calculated byreversing the sign of the integral value obtained by Expression (20). Asdescribed above, when the first inverse integral value Intr1 iscalculated, the first target curvature Cuemergency 1* versus(corresponding to) the elapsed time t from the point at which theapproaching vehicle is detected can be calculated based on the magnitudeof the first inverse integral value Intr1, the maximum value Cumax ofthe target curvature, and the maximum change in gradient Cu′max of thetarget curvature. In this way, under the restriction of the maximumvalue Cumax and the maximum change in gradient Cu′max, the drivingsupport ECU 10 calculates the first target curvature Cuemergency 1* forreturning the first integral value Int1, which is the integral value ofthe target curvature Cu* from the lane change start time t0 to the timet1 a, to zero in the shortest time.

The above is the description of calculation of the first yaw anglereturn target trajectory (the first target curvature Cuemergency 1).

In Step S64 of FIG. 6, the driving support ECU 10 sends an alarm to thedriver to inform him/her that the LCA is halfway ended and that anapproaching vehicle is detected at the same time as the calculation ofthe first yaw angle return target trajectory. For example, the drivingsupport ECU 10 drives the buzzer 13 to generate an alarm sound (forexample, a “beeping” sound), and transmits an LCA approach warningcommand to the meter ECU 30. This alarm sound is issued in the highestattention call level mode.

Upon receiving the LCA approach warning command, the meter ECU 30displays an LCA approach warning screen 31 d on the display unit 31 asshown in FIG. 12. On the LCA approach warning screen 31 d, thetrajectory Z (see FIG. 8), which had been displayed on the display unit31 until just before, is not displayed, and a blinking alarm mark GA isdisplayed so as to be parallel and adjacent to the white line displayGWL which is positioned on lane change direction side (the right side inthis example). The driver can recognize that the LCA is halfwayterminated and another vehicle is abnormally approaching the own vehicleon the target lane by the sounding of the buzzer 13 and the LCA approachwarning screen 31 d displayed on the display unit 31. In this case, analarm message may be generated by voice announcement. A vibrator (notshown) may be vibrated to issue an alarm to the driver. The LCA approachwarning screen 31 d continues to be displayed until a predeterminedcondition is established.

Upon completion of the processing of Step S64, the driving support ECU10 proceeds to Step S65 to set the original lane return flag to “1”.

Upon completing the processing of Step S65, the driving support ECU 10proceeds to Step S65A to execute first yaw angle return control guardprocess. That is, the driving support ECU 10 recalculates the firsttarget curvature Cuemergency 1* (the first target curvature functionCuemergency 1*(t)), which has been calculated, by using a first guard G1defined by the trapezoidal shape shown by the broken line in FIG. 11.

The first guard G1 is defined by one first steering angle guard CumaxG1corresponding to the maximum value Cumax and two first steering angularvelocity guards Cu′maxG1 corresponding to the maximum change in gradientCu′max. The first steering angle guard CumaxG1 corresponds to thesteering angle guard at lane change time, and the first steering angularvelocity guard Cu′maxG1 corresponds to the steering angular velocityguard at lane change time.

Then, the driving support ECU 10 uses the first steering angle guardCumaxG1 instead of the maximum value Cumax and uses the first steeringangular velocity guard Cu′maxG1 instead of the maximum change ingradient Cu′ to recalculate the first target curvature Cuemergency 1*.That is, the driving support ECU 10 calculates a first target curvatureCuemergency 1G* (a first target curvature function Cuemergency 1G*(t))which is the recalculated value of the first target curvatureCuemergency 1* (the first target curvature function Cuemergency 1*(t)).

However, in the present embodiment, the absolute value of the firststeering angle guard CumaxG1 is larger than the absolute value of themaximum value Cumax and the absolute value of the first steering angularvelocity guard Cu′maxG1 is larger than the absolute value of the maximumchange in gradient Cu′max. Therefore, the first target curvatureCuemergency 1G* (the first target curvature function Cuemergency 1G*(t))is the same as the first target curvature Cuemergency 1* (the firsttarget curvature function Cuemergency 1*(t)). In other words, the firsttarget curvature Cuemergency 1* (the first target curvature functionCuemergency 1*(t)) is not corrected by the first steering angle guardCumaxG1 and the first steering angular velocity guard Cu′maxG1.

When the absolute value of the maximum value Cumax is larger than theabsolute value of the first steering angle guard CumaxG1 and/or theabsolute value of the maximum change in gradient Cu′max is larger thanthe absolute value of the first steering angular velocity guardCu′maxG1, the first target curvature Cuemergency 1* (the first targetcurvature function Cuemergency 1*(t)) is corrected by the first guardG1. That is, the absolute value of the maximum value Cumax of the firsttarget curvature Cuemergency 1G* (the first target curvature functionCuemergency 1G*(t)) and/or the absolute value of the maximum change ingradient Cu′max are (is) smaller than the corresponding value of thefirst target curvature Cuemergency 1*.

As will be described later, the first yaw angle return control isfeedforward control, and in the first yaw angle return control, only thefirst term of the Expression (15) is used. That is, the first steeringangle guard CumaxG1 limits the first term of the Expression (15).

Next, in Step S66 of the routine illustrated in FIG. 6, the drivingsupport ECU 10 performs steering control based on the first targetcurvature function Cuemergency 1*(t) (the first target curvaturefunction Cuemergency 1G*(t)) calculated in the previous Step S65A. Inthis case, the driving support ECU 10 resets a clock timer t (the clocktimer t starts after being cleared to zero), and calculates the firsttarget curvature Cuemergency 1* at the current time point based on theelapsed time t from the time t1 a at which the approaching vehicle isdetected and the first target curvature function Cuemergency 1*(t). Thedriving support ECU 10 calculates a target steering angel θemergency* atthe current time point based on the first target curvature Cuemergency1* and the curvature Cu which is being detected by the camera sensor 12at the current time point. This target steering angel θemergency* isreferred to as first target control amount. The target steering angelθemergency* is, as shown in Expression (21), calculated by multiplyingthe control gain Klca1 by a value obtained by adding the first targetcurvature Cuemergency 1* at the current time point and the curvature Cuwhich is being detected by the camera sensor 12.

θemergency*=Klca1·(Cuemergency 1*+Cu)  (21)

The driving support ECU 10 transmits a steering command representing thetarget steering angel θemergency* to the EPS·ECU 20 each time the targetsteering angel θemergency* is calculated. When the EPS·ECU 20 receivesthe steering command, the EPS·ECU 20 controls the drive of the steeringmotor 22 so that the steering angle follows the target steering angelθemergency*. In this embodiment, the driving support ECU 10 calculatesthe target steering angel θemergency* as the target control amount, butthe driving support ECU 10 may calculate a target torque for obtainingthe target steering angel θemergency*, and transmit a steering commandrepresenting that target torque to the EPS·ECU 20.

In the following description, steering control using the target steeringangel θemergency* based on the first target curvature Cuemergency 1* isreferred to as first yaw angle return control. In the first yaw anglereturn control, the steering angle is controlled based only onfeed-forward control term using the value obtained by adding the firsttarget curvature Cuemergency 1* and the curvature Cu detected by thecamera sensor 12.

More specifically, feedback control using the yaw angle 9 y detected bythe camera sensor 12 is not performed. Noted that, also in second yawangle return control described later, feedback control using the yawangle θy detected by the camera sensor 12 is not performed.

The driving support ECU 10 may also store the values of the feedbackcontrol amounts (the second to fifth terms on the right-hand side ofExpression (15)) calculated immediately before the point (time t1 a) atwhich the approaching vehicle is detected, and during the first yawangle return control, may add those stored values (fixed values) to theright-hand side of Expression (21) as a part of the feed-forward controlamounts.

Next, in Step S67, the driving support ECU 10 determines whether or notthe first yaw angle return control is completed. The first yaw anglereturn control is completed at a timing (the time t3 a in FIG. 11) atwhich the first target curvature Cuemergency 1* becomes substantiallyzero. In the present embodiment, there is a difference between the timet2 a−1 at which the formation of the first inverse integral value Intr1is started and the time t1 a at which an approaching vehicle is detected(that is, the time at which the first interruption condition isestablished). However, since this difference is extremely small, the yawangle at the time t3 a becomes substantially the same as the yaw angleat the lane change start time t0. In the present embodiment, the timeperiod from the time t1 a to the time t3 a is referred to as a firstcontrol execution time TC1. When the current time has not reached thetime t3 a, the driving support ECU 10 returns the processing to Step S66and executes similar processing. By repeating such processing at apredetermined calculation cycle, the yaw angle is reduced at a highspeed.

Through repeating this processing, when the first yaw angle returncontrol is completed at the time t3 a (Step S67: Yes), the drivingsupport ECU 10 proceeds to Step S18 of the flowchart in FIG. 5.

The yaw angle is decreased to be substantially zero at the time t3 a.That is, lateral speed of the own vehicle is substantially zero.Therefore, the own vehicle can be controlled so as not to move to thecenter of the target lane in the lane width direction, and thuscollision with the approaching vehicle is avoided. This function part ofthe driving support ECU 10, which executes the first yaw angle returncontrol (Steps S64 through S67), corresponds to collision avoidancesupport controller of the present invention.

In Step S18, the driving support ECU 10 determines whether or not theoriginal lane return flag is “1”.

In this case, the driving support ECU 10 determines Yes in Step S18 toproceed to Step S19.

In Step S19, the driving support ECU 10 determines whether or not acollision time TTCr is equal to or more than a predetermined thresholdTTCth r.

The collision time TTCr is an estimated time period from the currenttime until the own vehicle collides with another vehicle on the originallane when the original lane return control is executed. The collisiontime TTCr is calculated by the driving support ECU 10 in the same manneras the above method to calculate the collision time TTC. For example,the threshold TTCthr is set to 4 seconds.

When determining “No” in Step S19, the drive support ECU 10 temporarilyends the processing of the flowchart of FIG. 5. That is, the drivingsupport ECU 10 temporarily ends the steering assist control.

When determining Yes in Step S19, the driving support ECU 10 proceeds toStep S70. FIG. 7 is a flowchart showing the processing of Step S70(subroutine B). The control indicated by the subroutine B is referred toas an original lane return control.

In Step S71, the driving support ECU 10 calculates a target trajectoryfor moving the own vehicle from the current position (the position ofthe own vehicle at the moment at which the first yaw angle returncontrol is completed) to the center position of the original lane.Hereinafter, this target trajectory is referred to as original lanereturn target trajectory. As for this original lane return targettrajectory, the function y(t) shown in the Expression (2) is also used.The function representing the original lane return target trajectory iscalled original lane return target trajectory function y(t). When theoriginal lane return target trajectory function y(t) is calculated, inorder to determine the coefficients c0, c1, c2, c3, c4, c5 of thefunction y(t) shown in the Expression (2), original lane return targettrajectory calculation parameters are set as follows. The original lanereturn target trajectory calculation parameters are the following sevenparameters (P21 to P27).

P21. Lateral position of the own vehicle at the current time (at thetime when the first yaw angle return control is completed)

P22. Lateral speed of the own vehicle at the current time (at the timewhen the first yaw angle return control is completed)

P23. Lateral acceleration of the own vehicle at the current time (at thetime when the first yaw angle return control is completed)

P24. Target lateral position which is the target value of the lateralposition and to which the own vehicle is moved (In this example, thetarget lateral position is the center position of the original lane, andhereinafter is referred to as original lane return completion targetlateral position.)

P25. Target lateral speed of the own vehicle when the own vehicle ismoved to the original lane return completion target lateral position(The target lateral speed is referred to as original lane returncompletion target lateral speed.)

P26. Target lateral acceleration of the own vehicle when the own vehicleis moved to the original lane return completion target lateral position(The target lateral acceleration is referred to as original lane returncompletion target lateral acceleration.)

P27. Target time period which is target value of time period requiredfor moving the own vehicle from the current position to the originallane return completion target lateral position (The target time periodis referred to as original lane return completion target time period.)

Here, it is assumed that the lateral position of the own vehicle at thecurrent time (when the first yaw angle return control is completed) isyreturn, the lateral speed is vyreturn, the lateral acceleration isayreturn, the time t at which the first yaw angle return control iscompleted is newly set to zero (t=0), and the original lane returncompletion target time period is treturn. The original lane returntarget trajectory calculation parameters are set as follows:y(0)=yreturn, y′(0)=vyreturn, y″(0)=ayreturn, y(treturn)=W (the sign isset according to the lane change direction), Y′(treturn)=0,y″(treturn)=0.

The lateral position yreturn, the lateral speed vyreturn, and thelateral acceleration ayreturn are detected values at the current time,and can be calculated in the same way as the above-described method forcalculating the initial lateral state amount. That is, the lateralposition yreturn is the lateral deviation Dy at the current time. Thelateral speed vyreturn can be obtained from the vehicle speed v at thecurrent time and the yaw angle θy at the current time (vyreturn=v·sin(θy)). The lateral acceleration ayreturn is the value (v·γ) obtained bymultiplying the yaw rate γ at the current time by the vehicle speed v atthe current time. The y(treturn) is the original lane return completiontarget lateral position, and is set to the center position of theoriginal lane. In this case, when the camera sensor 12 is outputting thelane information of the original lane at the time when the first yawangle return control is completed, y(treturn)=0. The y′(treturn)represents the original lane return completion target lateral speed, andthe y″(treturn) represents the original lane return completion targetlateral acceleration. Both the y′(treturn) and the y″(treturn) are setto zero.

In addition, the original lane return completion target time periodtreturn is calculated using a target time setting constant Areturn,which is approximately the same value as the target time settingconstant A used when the target lane change time tlen is calculated atthe start of the LCA, by the following Expression (22).

treturn=Dreturn·Areturn  (22)

Here, Dreturn is a necessary distance for moving the own vehicle in thelateral direction from the lateral position of the own vehicle at thetime when the first yaw angle return control is completed to theoriginal lane return completion target lateral position (the centerposition of the original lane). At the time when the first yaw anglereturn control is completed, a collision of the own vehicle with anothervehicle is avoided. Therefore, the speed at which the position of theown vehicle is moved in the lateral direction can be approximately thesame as the LCA, and thus the target time setting constant Areturn isset to a value approximately the same as the target time settingconstant A when the LCA is executed.

Based on the set values of the original lane return target trajectorycomputing parameter, the driving support ECU 10 calculates the values ofthe coefficients c0, c1, c2, c3, c4, c5 of the function y(t) shown bythe Expression (2) in the same way as Step S13. Then, by substitutingthe values of the calculated coefficients c0, c1, c2, c3, c4, c5 intothe Expression (2), the original lane return target trajectory functiony(t) is calculated.

Upon calculating the original lane return target trajectory function inStep S71, the driving support ECU 10 advances the processing to StepS72. In Step S72, the driving support ECU 10 performs steering controlbased on the original lane return target trajectory function calculatedin Step S71. In this case, the driving support ECU 10 resets the clocktimer t (the clock timer t starts after being cleared to zero), andcalculates the target lateral movement state quantity (y*, vy*, ay*) andthe target yaw state quantity (θy*, γ*, Cu*) based on the elapsed time tfrom the time when the first yaw angle return control is completed andthe original lane return target trajectory function y(t) in the samemanner as Step S14 to calculate the final target steering angelθreturn*. The target steering angle θreturn* can be calculated, forexample, by replacing the left-hand side of Expression (15) with thetarget steering angle θreturn*.

Upon calculating the target control amount (target steering angleθreturn*), the driving support ECU 10 transmits a steering commandrepresenting the target control amount to the EPS·ECU 20. In the presentembodiment, the driving support ECU 10 calculates the target steeringangle θreturn* as the target control amount. However, the drivingsupport ECU 10 may calculate a target torque for obtaining the targetsteering angle θreturn*, and may transmit the steering commandrepresenting the target torque to the EPS·ECU 20.

Subsequently, in Step S73, the driving support ECU 10 determines whetheror not an original lane return control end condition is established. Inthis case, when detecting that the lateral position of the own vehiclehas reached the original lane return completion target lateral position(the center position of the original lane) by the steering control inStep S72, the driving support ECU 10 determines that the original lanereturn control end condition is established.

When determining that the original lane return control end condition isnot established (Step S73: No), the driving support ECU 10 returns theprocessing to Step S72. Therefore, the steering control in Step S72continues to be executed until the original lane return control endcondition is established. As a result, the own vehicle travels towardthe center position of the original lane.

Through repeating this processing, when the original lane return controlend condition is established, the driving support ECU 10 ends theprocessing of the subroutine B of FIG. 7, and proceeds to Step S17 ofthe main routine (the steering assist control routine). As a result, thesteering assist control state is switched to the LTA ON state. Thefunction part of the driving support ECU 10 which executes theprocessing from Step S71 to Step S73 corresponds to the original lanereturn assist controller of the present invention.

The broken line in FIG. 13 represents the target lane return targettrajectory when the own vehicle C1 and another vehicle C3 approach eachother.

On the other hand, when the driving support ECU 10 determines No in StepS63 of the subroutine A in FIG. 6, the driving support ECU 10 proceedsto Step S68. That is, when the second interruption condition isestablished, the driving support ECU 10 proceeds to Step S68. In StepS68, the driving support ECU 10 sets the steering assist control stateto a second yaw angle return control state and terminates the LCA.

Further, in Step S68, the driving support ECU 10 calculates a second yawangle return target trajectory (see FIG. 13) for returning the yaw angleof the own vehicle to the state (the yaw angle) immediately before thestart of the LCA.

The calculation method of the second yaw angle return target trajectoryis similar to the calculation method of the first yaw angle returntarget trajectory.

For example, it is assumed that the second interruption condition isestablished at a time t1 b when a time t0 in FIG. 14 is the lane changestart time of the LCA. The second yaw angle return target trajectory isdefined by the target curvature versus (corresponding to) the elapsedtime from the time when the second interruption condition isestablished.

Also in this case, similarly to the first yaw angle return control, theintegral value of the target curvature Cu* from the lane change starttime t0 to the time t1 b corresponds to the surface area of the portioncolored in gray in FIG. 14. Therefore, when the sign of the feed-forwardcontrol amount corresponding to that surface area is reversed (theleft-right direction is reversed) and the feed-forward control amount isissued as a command to the EPS·ECU 20, the yaw angle can be returned tothe state at the lane change start time t0 at the point when output ofthe feed-forward control amount is complete. The value obtained byreversing the sign (plus or minus) of a second integral value Int2,which is the integral value of the target curvature Cu* from the lanechange start time t0 to the time t1 b, is referred to as a secondinverse integral value Intr2. This second inverse integral value Intr2corresponds to the surface area of a trapezoidal portion formed underthe abscissa axis (time axis) between a time t2 b−1 and a time t3 bwhich is the finish time of the control based on the second yaw anglereturn target trajectory in FIG. 14. The value obtained by adding thissecond inverse integral value Intr2 to the second integral value Int2 ofthe target curvature Cu* from the lane change start time t0 to the timet1 b is zero.

When the second yaw angle return target trajectory is calculated, asecond control execution time TC2 which is the time period between thetime t1 b and the time t3 b is set to be longer than the first controlexecution time TC1. Further, the time period from a time t2 b−1 to atime t2 b−2 which is the time corresponding to the start point of thehorizontal lower end portion of the trapezoidal shape (the secondinverse integral value Intr2) is set to TC2 a (for example, 0.5seconds). The time period from the time t2 b−2 to the time t2 b−3 whichis the time corresponding to the end point of the lower end portion ofthe trapezoidal shape is set to TC2 b (for example, 3.0 seconds).Further, the time period from time t2 b−3 to the time t3 b is set to TC2c (for example, 0.5 seconds).

The driving support ECU 10 calculates a second target curvatureCuemergency 2* which is a target curvature versus (corresponding to) theelapsed time t from the time t1 b by using the value of the secondinverse integral value Intr2, the TC2 a, the TC2 b, and the TC2 c. Inother words, the driving support ECU 10 calculates the maximum value(lower end position of the trapezoidal shape) of the second targetcurvature Cuemergency 2* based on the sum total of the TC2 a, the TC2 b,and the TC2 c which are the upper base of the second inverse integralvalue Intr2 (the trapezoidal shape) and the TC2 b which is the lowerbase of the second inverse integral value Intr2 (the trapezoidal shape)to determine the outer shape of the second integral value Int2. Further,the driving support ECU 10 calculates the second target curvatureCuemergency 2* between the time t1 b and the time t2 b−1 by extendingthe inclined straight line which is the outer shape of the secondintegral value Int2 between a time t2 b−1 and the time t2 b−2.

The maximum value of the second target curvature Cuemergency 2* whichcorresponds to the lower end portion of the trapezoidal shape calculatedin this manner is smaller than the maximum value (Cumax) of the firsttarget curvature Cuemergency 1*. Both the inclination of the secondtarget curvature Cuemergency 2* between the time t2 b−1 and the time t2b−2 and the inclination of the second target curvature Cuemergency 2*between the time t2 b−3 and the time t3 b are smaller than the maximumchange in gradient (Cu′max) of the first target curvature Cuemergency1*.

Hereinafter, the second target curvature Cuemergency 2* versus(corresponding to) the elapsed time t may be referred to as a secondtarget curvature function Cuemergency 2*(t). The second target curvaturefunction Cuemergency 2*(t) determines the target trajectory of the ownvehicle. Therefore, the second target curvature function Cuemergency2*(t) corresponds to the second yaw angle return target trajectory.

The above is a description of the calculation of the second yaw anglereturn target trajectory (the second target curvature Cuemergency 2*).

In Step S68 of FIG. 6, the driving support ECU 10 sends an alarm to thedriver to inform him/her that the LCA is halfway ended and that thecamera sensor 12 cannot (fails to) recognize the relative position ofthe own vehicle with respect to the lane in the lane width direction atthe same time as the calculation of the second yaw angle return targettrajectory.

Upon receiving a command from the driving support ECU 10 in Step S68,the meter ECU 30 displays a relative position unrecognized warningscreen 31 e on the display unit 31 as shown in FIG. 15. On the relativeposition unrecognized warning screen 31 e, the trajectory Z, which hadbeen displayed on the display unit 31 until just before, is notdisplayed.

Upon completion of the processing of Step S68, the driving support ECU10 proceeds to Step S69 to set the original lane return flag to “0”.

Upon completing the processing of Step S69, the driving support ECU 10proceeds to Step S69A to execute second yaw angle return control guardprocess. That is, the driving support ECU 10 recalculates the secondtarget curvature Cuemergency 2* (the second target curvature functionCuemergency 2*(t)), which has been calculated, by using a second guardG2 defined by the trapezoidal shape shown by the broken line in FIG. 14.

The second guard G2 is defined by one second steering angle guardCumaxG2 corresponding to the first steering angle guard CumaxG1 (and thesteering angle guard at lane change time) and two second steeringangular velocity guards Cu′maxG2 corresponding to the first steeringangular velocity guard Cu′maxG1 (and the steering angular velocity guardat lane change time).

Noted that, the absolute value of the second steering angle guardCumaxG2 is smaller than the absolute value of the first steering angleguard CumaxG1 and the absolute value of the second steering angularvelocity guard Cu′maxG2 is smaller than the absolute value of the firststeering angular velocity guard Cu′maxG1. Further, the absolute value ofthe second steering angle guard CumaxG2 is the same as the absolutevalue of the steering angle guard at lane change time, and the absolutevalue of the second steering angular velocity guard Cu′maxG2 is the sameas the absolute value of the steering angular velocity guard at lanechange time.

Then, the driving support ECU 10 uses the second steering angle guardCumaxG2 and the second steering angular velocity guard Cu′maxG2 torecalculate the second target curvature Cuemergency 2*. That is, thedriving support ECU 10 calculates a second target curvature Cuemergency2G* (a second target curvature function Cuemergency 2G*(t)) which is therecalculated value of the second target curvature Cuemergency 2* (thesecond target curvature function Cuemergency 2*(t)).

In the example shown by FIG. 14, the absolute value of the secondsteering angle guard CumaxG2 is smaller than the absolute value of themaximum value of the second target curvature Cuemergency 2*, and theabsolute value of the second steering angular velocity guard Cu′maxG2 issmaller than the absolute value of the maximum change in gradient of thesecond target curvature Cuemergency 2*. Therefore, at each time exceptfor the time t2 b−1 and the time t3 b, the absolute value of the maximumvalue of the second target curvature Cuemergency 2G* and the absolutevalue of the maximum value of the target steering angular velocityobtained from the second target curvature Cuemergency 2G* are smallerthan the absolute value of the maximum value of the second targetcurvature Cuemergency 2* and the absolute value of the maximum value ofthe target steering angular velocity obtained from the second targetcurvature Cuemergency 2*, respectively.

When the absolute value of the second steering angle guard CumaxG2 islarger than the absolute value of the maximum value of the second targetcurvature Cuemergency 2*, and the absolute value of the second steeringangular velocity guard Cu′maxG2 is larger than the absolute value of themaximum change in gradient of the second target curvature Cuemergency2*, the second target curvature Cuemergency 2* (the second targetcurvature function Cuemergency 2*(t)) is not corrected by the secondsteering angle guard CumaxG2 and the second steering angular velocityguard Cu′maxG2.

As described above, the second yaw angle return control is feedforwardcontrol, and in the second yaw angle return control, only the first termof the Expression (15) is used. That is, the second steering angle guardCumaxG2 limits the first term of the Expression (15).

Next, in Step S66, the driving support ECU 10 performs steering controlbased on the second target curvature Cuemergency 2G* calculated in theprevious Step S69A. This steering control based on the second targetcurvature Cuemergency 2G* is substantially the same as the steeringcontrol based on the first target curvature function Cuemergency 1*(t).That is, the driving support ECU 10 resets the clock timer t (the clocktimer t starts after being cleared to zero), and calculates the secondtarget curvature Cuemergency 2G* at the current time point based on theelapsed time t from the time t1 b at which the camera sensor 12 failedto recognize the relative position of the own vehicle and the secondtarget curvature function Cuemergency 2G*(t).

The driving support ECU 10 calculates a target steering angelθemergency* at the current time point based on the second targetcurvature Cuemergency 2G* and the curvature Cu which is the latest oneamong the curvatures Cu detected by the camera sensor 12 before the timet1 b. This target steering angel θemergency* is referred to as secondtarget control amount.

Further, the driving support ECU 10 transmits a steering commandrepresenting the target steering angel θemergency* to the EPS·ECU 20each time the target steering angel θemergency* is calculated. When theEPS·ECU 20 receives the steering command, the EPS·ECU 20 controls thedrive of the steering motor 22 so that the steering angle follows thetarget steering angel θemergency*.

In the following description, steering control using the target steeringangel θemergency* based on the second target curvature Cuemergency 2G*is referred to as second yaw angle return control. Also in the secondyaw angle return control, the steering angle is controlled based only onfeed-forward control term using the value obtained by adding the secondtarget curvature Cuemergency 2G* and the curvature Cu detected by thecamera sensor 12.

Next, in Step S67, the driving support ECU 10 determines whether or notthe second yaw angle return control is completed. The second yaw anglereturn control is completed at a timing (the time t3 b in FIG. 14) atwhich the second target curvature Cuemergency 2G* becomes zero. There isa difference between the time t2 b−1 at which the formation of thesecond inverse integral value Intr2 is started and the time t1 b atwhich the camera sensor 12 became unable to recognize the relativeposition of the own vehicle in the lane width direction (that is, thetime at which the second interruption condition is established).Further, there is a difference between the second target curvatureCuemergency 2* and the second target curvature Cuemergency 2G*. However,since these differences are extremely small, the yaw angle at thecompletion time of the second yaw angle return control becomessubstantially the same as the yaw angle at the lane change start timet0.

When the second yaw angle return control is completed (Step S67: Yes),the driving support ECU 10 proceeds to Step S18 of the flowchart of FIG.5, and determines No in Step S18.

Then, the driving support ECU 10 temporarily ends the processing of theflowchart of FIG. 5. That is, the driving support ECU 10 temporarilyends the steering assist control.

In this case, for example, the steering assist control is temporarilyended while the own vehicle is positioned on the target lane. In otherwords, there is a fear of the driver feeling that the steering assistcontrol has been suddenly ended. Therefore, there is a fear that thedriver can not properly steer the steering wheel immediately uponcompletion of the steering assist control.

However, at the time t3 b which is the finish time of the second yawangle return control, the yaw angle is reduced to almost zero.Therefore, since the own vehicle does not move to the center of thetarget lane in the width direction, there is no possibility that the ownvehicle collides with an approaching vehicle.

According to the steering assist device of the present embodimentdescribed above, the first yaw angle return control is executed when thefirst interruption condition is established during the LCA, and thesecond yaw angle return control is executed when the second interruptioncondition is established during the LCA. Then, the own vehicle isprevented from moving to the center of the target lane in the widthdirection by executing the first yaw angle return control or the secondyaw angle return control.

Incidentally, the first interruption condition is established when thecollision time TTC from the current time until another vehicle collideswith the own vehicle is less than the threshold TTCth under the statewhere the LCA is executed. That is, the first yaw angle return controlis executed when the own vehicle is likely to collide with anothervehicle under the state where the own vehicle continues the execution ofthe LCA until the LCA completion condition is established.

Therefore, when the first yaw angle return control is executed, it isnecessary to quickly return the yaw angle of the own vehicle to the samevalue or substantially the same value as the yaw angle at the lanechange start time t0.

Therefore, the first control execution time TC1 of the first yaw anglereturn control is set to be shorter than the second control executiontime TC2 of the second yaw angle return control.

On the other hand, the second interruption condition is established whenthe camera sensor 12 becomes unable to (fails to) recognize the relativeposition of the own vehicle in the lane width direction. In other words,in the case where the second interruption condition is established, theown vehicle is unlikely to collide with another vehicle under the statewhere the own vehicle continues the execution of the LCA until the LCAcompletion condition is established.

Therefore, in this case, there is no need to quickly return the yawangle of the own vehicle to the same value or substantially the samevalue as the yaw angle at the lane change start time to.

Therefore, the second control execution time TC2 of the second yaw anglereturn control is set to be longer than the first control execution timeTC1 of the first yaw angle return control.

Therefore, the change rate per unit time of the yaw angle of the ownvehicle during the second yaw angle return control is smaller than thechange rate per unit time of the yaw angle of the own vehicle during thefirst yaw angle return control. Therefore, an occupant of the ownvehicle hardly feels uncomfortable during the second yaw angle returncontrol.

As described above, according to the present embodiment, it is possibleto return the yaw angle of the own vehicle to the same value orsubstantially the same value as the yaw angle at the lane change starttime t0 in an appropriate manner depending on the interruption factor ofthe LCA.

Incidentally, the camera sensor 12 of the own vehicle acquires the yawangle. That is, the camera sensor 12 can acquire the yaw angle at thelane change start time and the yaw angle at the time t1 a which is thestart time of the first yaw angle return control.

Therefore, it is theoretically possible to bring (operate) the steeringmotor 22 under feedforward control or feedback control so as to make avalue of the yaw angle at the finish time of the first yaw angle returncontrol be the same or substantially the same as the yaw angle at thelane change start time t0 when, for example, the yaw angle acquired bythe camera sensor 12 at time t1 a is greater than zero.

However, the camera sensor 12 acquires the yaw angle by photographingthe white line(s) WL, performing image processing on the acquiredimaging data, and calculating based on the image processed data.

That is, a predetermined time period which is too long to be ignoredelapses from when the camera sensor 12 captures the white line(s) WLuntil when the camera sensor 12 calculates the yaw angle. In otherwords, an error having a magnitude which is too large to be ignoredoccurs between the yaw angle acquired by the camera sensor 12 at thetime t1 a and the actual yaw angle at the time t1 a. Therefore, when thefirst yaw angle return control is executed in this way, there is a highpossibility that the yaw angle at the first finish time of the first yawangle return control does not become the same value or substantially thesame value as the yaw angle at the lane change start time t0.

Thus, in the first yaw angle return control of the present embodiment,the yaw angle is returned to the same value or substantially the samevalue as the yaw angle at the lane change start time t0 by thefeedforward control which is based on the integral value (the firstintegral value Int1) of the target curvature Cu* from the lane changestart time t0 to the time (t1 a) at which the first interruptioncondition is established.

This integral value does not include the above error caused by thecamera sensor 12. Therefore, the first yaw angle return control can beaccurately executed so that the yaw angle at time t3 a, which is thefinish time of the first yaw angle return control, is the same value orsubstantially the same value as the yaw angle at the lane change starttime t0.

Further, similarly to the first yaw angle return control, the second yawangle return control is also executed with high accuracy so that the yawangle at the time t3 b, which is the finish time, is the same value orsubstantially the same value as the yaw angle at the lane change starttime t0.

It should be noted that the feed forward control amount in the first yawangle return control and the second yaw angle return control includesthe component (Klca1·Cu) of the curvature Cu representing the curveshape of the road. However, since this component is control amount forcausing the own vehicle to travel along the road shape and the degree ofthe change of this component is extremely gentle, this component doesnot adversely affect on the yaw angle return control.

Then, the absolute value of the first steering angle guard CumaxG1 andthe absolute value of the first steering angular velocity guardCu′maxG1, which are guards for the first yaw angle return controlexecuted when a probability of the own vehicle colliding with anothervehicle is high, are set to a value larger than the absolute value ofthe second steering angle guard CumaxG2 (the steering angle guard atlane change time) and a value larger than the absolute value of thesecond steering angular velocity guard Cu′maxG2 (the steering angularvelocity guard at lane change time), which are guards for the second yawangle return control executed when the probability of occurrence of thecollision is not high, respectively.

Thus, when the first yaw angle return control is executed, the targetsteering angle θlca* and the target steering angular velocity don'tbecome small values. Therefore, the first yaw angle return control canbe quickly executed.

Meanwhile, the second steering angle guard CumaxG2 and the secondsteering angular velocity guard Cu′maxG2, which are guards for thesecond yaw angle return control executed when the probability ofoccurrence of the collision is not high, are smaller than the firststeering angle guard CumaxG1 and the first steering angular velocityguard Cu′maxG1, respectively.

Thus, when the second yaw angle return control is executed, the targetsteering angle θlca* and the target steering angular velocity don'tbecome large values. Therefore, during execution of the second yaw anglereturn control, ride comfort of an occupant of the own vehicle is hardto get worse.

In the above, the steering assist device according to the presentembodiment has been described, but the present invention is not limitedto the above-mentioned embodiment, and various changes are possiblewithin the range not departing from the object of the present invention.

For example, the absolute value of the first steering angle guardCumaxG1 may be smaller than the absolute value of the maximum valueCumax. Similarly, the absolute value of the first steering angularvelocity guard Cu′maxG1 may be smaller than the absolute value of themaximum change in gradient Cu′max.

However, also in this case, the absolute value of the first steeringangle guard CumaxG1 is made larger than the absolute value of the secondsteering angle guard CumaxG2 (and the steering angle guard at lanechange time), and the absolute value of the first steering angularvelocity guard Cu′maxG1 is made larger than the absolute value of thesecond steering angular velocity guard Cu′maxG2 (and the steeringangular velocity guard at lane change time).

The absolute value of the steering angle guard at lane change time maybe different from the absolute value of the second steering angle guardCumaxG2.

Further, the absolute value of the steering angular velocity guard atlane change time may be different from the absolute value of the secondsteering angular velocity guard Cu′maxG2.

During execution of the LTA, the steering motor 22 may be controlled byusing a steering angle guard for LTA for limiting the maximum value ofthe steering angle during the LTA and a steering angular velocity guardfor LTA for limiting the maximum value of the steering angular velocityduring the LTA.

Then, the absolute value of the steering angle guard for LTA may be thesame as the absolute value of the second steering angle guard CumaxG2.

The absolute value of the steering angle guard for LTA may be differentfrom the absolute value of the second steering angle guard CumaxG2.

The absolute value of the steering angular velocity guard for LTA may bethe same as the absolute value of the second steering angular velocityguard Cu′maxG2.

The absolute value of the steering angular velocity guard for LTA may bedifferent from the absolute value of the second steering angularvelocity guard Cu′maxG2.

Further, when the driving support ECU 10 calculates a target torque fromwhich the target steering angle θlca* is obtained and transmits thesteering command representing this target torque to the EPS·ECU 20, thepresent invention may be carried out in a following modified embodiment.

That is, the torque of the steering wheel when the lane change assistcontrol is executed is limited by a torque guard at lane change timethat defines the upper limit value of the torque, and torque change rateof the steering wheel when the lane change assist control is executed islimited by a torque change rate guard at lane change time defining theupper limit value of a torque change rate which is a change amount oftorque corresponding to the steering angular velocity per unit time.

Further, a first target torque and a first target torque change rate,which are target values of the torque and the torque change rate forexecuting the first yaw angle return control respectively, arecalculated.

Then, the first target torque is limited by a first torque guard thatdefines the upper limit value of the torque of the steering wheel and islarger than the torque guard at lane change time. The first targettorque change rate is limited by a first torque change rate guard thatdefines the upper limit value of the torque change rate and is largerthan the torque change rate guard at lane change time.

Both the steering angle guard at lane change time and the torque guardat lane change time are examples of “a steering angle correspondencevalue guard at lane change time”, and both the steering angular velocityguard at lane change time and the torque change rate guard at lanechange time are examples of “a steering angular velocity correspondencevalue guard at lane change time”.

Furthermore, both the first steering angle guard and the first torqueguard are examples of “a first steering angle correspondence valueguard”, and both the first steering angular velocity guard and the firsttorque change rate guard are examples of “a first steering angularvelocity correspondence value guard”.

During the original lane return control the steering motor 22 may becontrolled by using both a steering angle correspondence value guard fororiginal lane return control for limiting the maximum value of thesteering angle or the torque during the original lane return control anda steering angular velocity correspondence value guard for original lanereturn control for limiting the maximum value of the steering angularspeed or the torque change rate.

In this case, the absolute value of the steering angle correspondencevalue guard for original lane return control is made smaller than theabsolute value of the first steering angle correspondence value guard,and the absolute value of the steering angular velocity correspondencevalue guard for original lane return control is made smaller than theabsolute value of the first steering angular velocity correspondencevalue guard.

However, when the original lane return control is executed immediatelyafter the completion of the first yaw angle return control, in theinitial stage of the original lane return control, the absolute value ofthe steering angle correspondence value guard for original lane returncontrol may temporarily set to be the same as the absolute value of thefirst steering angle correspondence value guard and the absolute valueof the steering angular velocity correspondence value guard for originallane return control may temporarily set to be the same as the absolutevalue of the first steering angular velocity correspondence value guard.As a result, when the original lane return control is executedimmediately after the completion of the first yaw angle return control,the occupant of the own vehicle is hard to feel uncomfortable.

In the original lane return control of the above embodiment, the finaltarget lateral position is set to the center position of the originallane. However, it is not always necessary to set the final targetlateral position to the center position. That is, for example, the finaltarget lateral position may be set to a predetermined lateral positionwithin the original lane.

For example, in the embodiment, the LCA is executed on the presumptionthat the steering assist control state is the LTA-ON state (state inwhich the LTA is executed), but such presumption is not necessarilyrequired. The presumption that ACC is being executed is also notrequired. In this embodiment, the LCA is executed on the condition thatthe road along which the own vehicle travels is a road for exclusive useby automobiles, but it is not always required for that condition to beincluded.

In the above embodiment, the lane is recognized by the camera sensor 12.However, for example, the navigation ECU 70 may detect the relativeposition of the own vehicle with respect to the lane.

What is claimed is:
 1. A lane change assist device comprising: asurrounding monitor configured to monitor a surrounding of a ownvehicle; a lane recognition device configured to recognize a compartmentline defining a side edge portion of a lane on which the own vehicle istraveling, and to detect a relative position of the own vehicle in alane width direction with respect to the lane on which the own vehicleis traveling and detect a yaw angle with respect to an extensiondirection of the lane on which the own vehicle is traveling based on apositional relationship between the compartment line and the ownvehicle; an actuator configured to be capable of generating a drivingforce for changing a steering angle correspondence value which is asteering angle of a steering wheel of the own vehicle or is a torquecorresponding to the steering angle and changing a steering angularvelocity correspondence value which is a steering angular velocity beinga change amount of the steering angle per unit time or a torque changerate being a change amount of the torque corresponding to the steeringangular velocity per unit time; a lane change assist controllerconfigured to start lane change assist control at a predetermined lanechange start time, wherein the actuator is controlled under the lanechange assist control so that the own vehicle makes a lane change froman original lane on which the own vehicle is traveling to a target lanewhich is adjacent to the original lane based on the relative positiondetected by the lane recognition device; a limiter at lane change timeconfigured to limit the steering angle correspondence value when thelane change assist control is executed by a steering anglecorrespondence value guard at lane change time defining an upper limitvalue of the steering angle correspondence value and to limit thesteering angular velocity correspondence value when the lane changeassist control is executed by a steering angular velocity correspondencevalue guard at lane change time defining an upper limit value of thesteering angular velocity correspondence value; a first interruptioncondition determiner configured to make the lane change assistcontroller interrupt the lane change assist control when a predeterminedfirst interruption condition is established after the lane change assistcontrol is started, the first interruption condition being establishedwhen it is determined that a probability of the own vehicle collidingwith another vehicle travelling on the target lane is high based on amonitoring result of the surrounding monitor; a first target valuecalculator configured to calculate a first target steering anglecorrespondence value which is a target value of the steering anglecorrespondence value and a first target steering angular velocitycorrespondence value which is a target value of the steering angularvelocity correspondence value, both the first target steering anglecorrespondence value and the first target steering angular velocitycorrespondence value being used for executing first yaw angle returncontrol, the first yaw angle return control being started at apredetermined first start time when the first interruption condition isestablished, wherein the actuator is controlled under the first yawangle return control so that the yaw angle at a first finish timebecomes a value closer to the yaw angle at the lane change start timecompared with the yaw angle at the first start time, the first finishtime coming when a predetermined first control execution time passesfrom the first start time; a first limiter configured to limit the firsttarget steering angle correspondence value by a first steering anglecorrespondence value guard which defines the upper limit value of thesteering angle correspondence value and is larger than the steeringangle correspondence value guard at lane change time and to limit thefirst target steering angular velocity correspondence value by a firststeering angular velocity correspondence value guard which defines theupper limit value of the steering angular velocity correspondence valueand is larger than the steering angular velocity correspondence valueguard at lane change time; and an actuator controller for first yawangle return control configured to control the actuator to operate thesteering wheel so that the steering angle correspondence value becomesthe first target steering angle correspondence value and the steeringangular velocity correspondence value becomes the first target steeringangular velocity correspondence value, wherein the first target steeringangle correspondence value and the first target steering angularvelocity correspondence value are limited by the first limiter.
 2. Thelane change assist device according to claim 1, further comprising: asecond interruption condition determiner configured to make the lanechange assist controller interrupt the lane change assist control when apredetermined second interruption condition is established after thelane change assist control is started, the second interruption conditionbeing established when the lane recognition device cannot detect therelative position; a second target value calculator configured tocalculate a second target steering angle which is a target value of thesteering angle and a second target steering angular velocity which is atarget value of the steering angular velocity, both the second targetsteering angle and the second target steering angular velocity beingused for executing second yaw angle return control, the second yaw anglereturn control being started at a predetermined second start time whenthe second interruption condition is established, wherein the actuatoris controlled under the second yaw angle return control so that the yawangle at a second finish time becomes a value closer to the yaw angle atthe lane change start time compared with the yaw angle at the secondstart time, the second finish time coming when a predetermined secondcontrol execution time which is longer than the first control executiontime passes from the second start time; a second limiter configured tolimit the second target steering angle by a second steering angle guardwhich defines an upper limit value of the steering angle and is smallerthan the first steering angle correspondence value guard and to limitthe second target steering angular velocity by a second steering angularvelocity guard which defines an upper limit value of the steeringangular velocity and is smaller than the first steering angular velocitycorrespondence value guard; and an actuator controller for second yawangle return control configured to control the actuator to operate thesteering wheel so that the steering angle becomes the second targetsteering angle and the steering angular velocity becomes the secondtarget steering angular velocity, wherein the second target steeringangle and the second target steering angular velocity are limited by thesecond limiter.